Fresh macOS install errors out on import
# 🐛 Bug
Fresh Install errors out on import

## Information
Following from the README instructions, created a venv, installed torch and tensorflow. Installed transformers. Upon import 

Model I am using (Bert, XLNet ...):
transformers==2.5.1

Language I am using the model on (English, Chinese ...):
N/A
The problem arises when using:
* [X] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [X] an official GLUE/SQUaD task: (give the name)
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. Create a new virtual env ```$ virtualenv -p python3 venv```
2. Source your new venv ```$ source venv/usr/local/bin/activate```
3. Pip install packages ``` (venv) $ pip install torch tensorflow transformers```
4. import package ``` python -m transformers```

```python

>>> import transformers
Traceback (most recent call last):
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/runpy.py", line 183, in _run_module_as_main
    mod_name, mod_spec, code = _get_module_details(mod_name, _Error)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/runpy.py", line 142, in _get_module_details
    return _get_module_details(pkg_main_name, error)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/runpy.py", line 109, in _get_module_details
    __import__(pkg_name)
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/transformers/__init__.py", line 22, in <module>
    from .configuration_albert import ALBERT_PRETRAINED_CONFIG_ARCHIVE_MAP, AlbertConfig
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/transformers/configuration_albert.py", line 18, in <module>
    from .configuration_utils import PretrainedConfig
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/transformers/configuration_utils.py", line 25, in <module>
    from .file_utils import CONFIG_NAME, cached_path, hf_bucket_url, is_remote_url
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/transformers/file_utils.py", line 53, in <module>
    import tensorflow as tf
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/tensorflow/__init__.py", line 101, in <module>
    from tensorflow_core import *
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/tensorflow_core/__init__.py", line 40, in <module>
    from tensorflow.python.tools import module_util as _module_util
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 959, in _find_and_load_unlocked
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/tensorflow/__init__.py", line 50, in __getattr__
    module = self._load()
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/tensorflow/__init__.py", line 44, in _load
    module = _importlib.import_module(self.__name__)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/tensorflow_core/python/__init__.py", line 64, in <module>
    from tensorflow.core.framework.graph_pb2 import *
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/tensorflow_core/core/framework/graph_pb2.py", line 7, in <module>
    from google.protobuf import descriptor as _descriptor
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/google/protobuf/__init__.py", line 37, in <module>
    __import__('pkg_resources').declare_namespace(__name__)
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/pkg_resources/__init__.py", line 84, in <module>
    __import__('pkg_resources.extern.packaging.requirements')
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/pkg_resources/_vendor/packaging/requirements.py", line 9, in <module>
    from pkg_resources.extern.pyparsing import stringStart, stringEnd, originalTextFor, ParseException
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 668, in _load_unlocked
  File "<frozen importlib._bootstrap>", line 638, in _load_backward_compatible
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/pkg_resources/extern/__init__.py", line 43, in load_module
    __import__(extant)
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/pkg_resources/_vendor/pyparsing.py", line 4756, in <module>
    _escapedPunc = Word( _bslash, r"\[]-*.$+^?()~ ", exact=2 ).setParseAction(lambda s,l,t:t[0][1])
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/pkg_resources/_vendor/pyparsing.py", line 1284, in setParseAction
    self.parseAction = list(map(_trim_arity, list(fns)))
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/pkg_resources/_vendor/pyparsing.py", line 1066, in _trim_arity
    this_line = extract_stack(limit=2)[-1]
  File "/Users/kiran/Library/Python/3.7/lib/python/site-packages/pkg_resources/_vendor/pyparsing.py", line 1050, in extract_stack
    frame_summary = traceback.extract_stack(limit=-offset+limit-1)[offset]
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/traceback.py", line 211, in extract_stack
    stack = StackSummary.extract(walk_stack(f), limit=limit)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/traceback.py", line 363, in extract
    f.line
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/traceback.py", line 285, in line
    self._line = linecache.getline(self.filename, self.lineno).strip()
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/linecache.py", line 16, in getline
    lines = getlines(filename, module_globals)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/linecache.py", line 48, in getlines
    for mod in sys.modules.values():
RuntimeError: dictionary changed size during iteration
```

## Expected behavior

Importing transformers into the python runtime should import transformers. 

- `transformers` version: 2.5.1
- Platform: macOS Catalina 10.15.3 
- Python version: Python 3.7.3
- PyTorch version (GPU?): N/A
- Tensorflow version (GPU?): N/A
- Using GPU in script?: N/A
- Using distributed or parallel set-up in script?: N/A

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
