text: Title: Cannot use gensim 3.8.x when `nltk` package is installed
  Body: #### Problem description
  
  > What are you trying to achieve? What is the expected result? What are you seeing instead?
  
  In my script i'm trying to `import gensim.models.keyedvectors` and also import another package, that requires `nltk` package internally. Whenever i have NLTK installed in the same virtualenv (i'm not using virtualenv, but a docker image actually) - the gensim model fails to import.
  
  #### Steps/code/corpus to reproduce
  
  ```
  # pip list | grep -E 'gensim|nltk'
  gensim                        3.8.1
  
  # pip install nltk
  Processing /root/.cache/pip/wheels/96/86/f6/68ab24c23f207c0077381a5e3904b2815136b879538a24b483/nltk-3.4.5-cp36-none-any.whl
  Requirement already satisfied: six in /usr/local/lib/python3.6/site-packages (from nltk) (1.13.0)
  Installing collected packages: nltk
  Successfully installed nltk-3.4.5
  
  # pip list | grep -E 'gensim|nltk'
  gensim                        3.8.1
  nltk                          3.4.5
  
  # python
  Python 3.6.8 (default, Jun 11 2019, 01:16:11)
  [GCC 6.3.0 20170516] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  >>> from gensim.models import word2vec;print("FAST_VERSION", word2vec.FAST_VERSION)
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "/usr/local/lib/python3.6/site-packages/gensim/__init__.py", line 5, in <module>
      from gensim import parsing, corpora, matutils, interfaces, models, similarities, summarization, utils  # noqa:F401
    File "/usr/local/lib/python3.6/site-packages/gensim/corpora/__init__.py", line 14, in <module>
      from .wikicorpus import WikiCorpus  # noqa:F401
    File "/usr/local/lib/python3.6/site-packages/gensim/corpora/wikicorpus.py", line 539, in <module>
      class WikiCorpus(TextCorpus):
    File "/usr/local/lib/python3.6/site-packages/gensim/corpora/wikicorpus.py", line 577, in WikiCorpus
      def __init__(self, fname, processes=None, lemmatize=utils.has_pattern(), dictionary=None,
    File "/usr/local/lib/python3.6/site-packages/gensim/utils.py", line 1614, in has_pattern
      from pattern.en import parse  # noqa:F401
    File "/usr/local/lib/python3.6/site-packages/pattern/text/en/__init__.py", line 61, in <module>
      from pattern.text.en.inflect import (
    File "/usr/local/lib/python3.6/site-packages/pattern/text/en/__init__.py", line 80, in <module>
      from pattern.text.en import wordnet
    File "/usr/local/lib/python3.6/site-packages/pattern/text/en/wordnet/__init__.py", line 57, in <module>
      nltk.data.find("corpora/" + token)
    File "/usr/local/lib/python3.6/site-packages/nltk/data.py", line 673, in find
      return find(modified_name, paths)
    File "/usr/local/lib/python3.6/site-packages/nltk/data.py", line 660, in find
      return ZipFilePathPointer(p, zipentry)
    File "/usr/local/lib/python3.6/site-packages/nltk/compat.py", line 228, in _decorator
      return init_func(*args, **kwargs)
    File "/usr/local/lib/python3.6/site-packages/nltk/data.py", line 506, in __init__
      zipfile = OpenOnDemandZipFile(os.path.abspath(zipfile))
    File "/usr/local/lib/python3.6/site-packages/nltk/compat.py", line 228, in _decorator
      return init_func(*args, **kwargs)
    File "/usr/local/lib/python3.6/site-packages/nltk/data.py", line 1055, in __init__
      zipfile.ZipFile.__init__(self, filename)
    File "/usr/local/lib/python3.6/zipfile.py", line 1131, in __init__
      self._RealGetContents()
    File "/usr/local/lib/python3.6/zipfile.py", line 1198, in _RealGetContents
      raise BadZipFile("File is not a zip file")
  zipfile.BadZipFile: File is not a zip file
  ```
  
  #### Versions
  
  ```python
   python
  Python 3.6.8 (default, Jun 11 2019, 01:16:11)
  [GCC 6.3.0 20170516] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  >>> import platform; print(platform.platform())
  Linux-5.0.0-050000rc8-generic-x86_64-with-debian-9.11
  >>> import sys; print("Python", sys.version)
  Python 3.6.8 (default, Jun 11 2019, 01:16:11)
  [GCC 6.3.0 20170516]
  >>> import numpy; print("NumPy", numpy.__version__)
  NumPy 1.17.4
  >>> import scipy; print("SciPy", scipy.__version__)
  SciPy 1.3.3
  >>> import gensim; print("gensim", gensim.__version__)
  gensim 3.8.1
  >>> from gensim.models import word2vec;print("FAST_VERSION", word2vec.FAST_VERSION)
  FAST_VERSION 1
  ```
  