seq2seq examples require pytest
## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 3.0.2
- Platform: Linux-5.4.0-42-generic-x86_64-with-glibc2.29
- Python version: 3.8.2
- PyTorch version (GPU?): 1.6.0 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: n/a
- Using distributed or parallel set-up in script?: n/a

### Who can help
 examples/seq2seq: @sshleifer
documentation: @sgugger

## To reproduce

Steps to reproduce the behavior:

1. Create a new virtual environment and set it up to run the examples tests. Do _not_ install `pytest` and `pytest-xdist`.
2. Run the tests with `unittest` as [described in the docs](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#tests)

## Expected behavior
The examples tests pass. Actual behavior:

```sh
======================================================================
ERROR: seq2seq.test_bash_script (unittest.loader._FailedTest)
----------------------------------------------------------------------
ImportError: Failed to import test module: seq2seq.test_bash_script
Traceback (most recent call last):
  File "/usr/lib/python3.8/unittest/loader.py", line 436, in _find_test_path
    module = self._get_module_from_name(name)
  File "/usr/lib/python3.8/unittest/loader.py", line 377, in _get_module_from_name
    __import__(name)
  File "/home/dmlap/projects/transformers/examples/seq2seq/test_bash_script.py", line 8, in <module>
    import pytest
ModuleNotFoundError: No module named 'pytest'


======================================================================
ERROR: seq2seq.test_seq2seq_examples (unittest.loader._FailedTest)
----------------------------------------------------------------------
ImportError: Failed to import test module: seq2seq.test_seq2seq_examples
Traceback (most recent call last):
  File "/usr/lib/python3.8/unittest/loader.py", line 436, in _find_test_path
    module = self._get_module_from_name(name)
  File "/usr/lib/python3.8/unittest/loader.py", line 377, in _get_module_from_name
    __import__(name)
  File "/home/dmlap/projects/transformers/examples/seq2seq/test_seq2seq_examples.py", line 10, in <module>
    import pytest
ModuleNotFoundError: No module named 'pytest'


----------------------------------------------------------------------
Ran 16 tests in 179.454s

FAILED (errors=2)
```

Perhaps the documentation should be updated to require `pytest`?
yes, great catch. Will update!