enable smart_open to operate without docstrings

[mpenkov]

Motivation

Fix #396

The below command was causing a crash:

$ python -OO -c 'import smart_open'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/misha/git/smart_open/smart_open/__init__.py", line 27, in <module>
    from .smart_open_lib import open, smart_open, register_compressor
  File "/home/misha/git/smart_open/smart_open/smart_open_lib.py", line 368, in <module>
    doctools.extract_kwargs(smart_open_s3.open.__doc__),
  File "/home/misha/git/smart_open/smart_open/doctools.py", line 66, in extract_kwargs
    lines = inspect.cleandoc(docstring).split('\n')
  File "/usr/lib/python3.6/inspect.py", line 620, in cleandoc
    lines = doc.expandtabs().split('\n')
AttributeError: 'NoneType' object has no attribute 'expandtabs'

The present PR fixes the underlying issue.

Checklist

Before you create the PR, please make sure you have:

• Picked a concise, informative and complete title
• Clearly explained the motivation behind the PR
• Linked to any existing issues that your PR will be solving
• Included tests for any new functionality
• Checked that all unit tests pass

[piskvorky]

I'm not familiar with that -00 mode or doctools workarounds, but the code itself looks fine 👍