Check-python-versions

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- Add ``pre-commit`` integration. See README file for instructions.

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- Recognize ``setuptools.setup()`` calls in setup.py.

- When parsing setup.py fails and check-python-versions falls back to running
``python setup.py --classifiers``, it will try to use python3, falling back
to python, and then to whatever interpreter was used to run
check-python-versions.

- Preserve formatting in ``python_requires``, e.g. when dropping 3.5,
``python_requires='>= 3.5'`` now becomes ``python_requires='>= 3.6'``
instead of ``python_requires='>=3.6'``.

-------------------

- When check-python-versions falls back to executing python setup.py, it
redirects the stdin to /dev/null, to prevent freezing in case setup.py
decides to ask the user some questions.

- Better error reporting in some obscure corner cases while parsing setup.py.

- The codebase now has docstrings and type hints, and passes mypy --strict.

- When tox.ini has an environment named 'pylint' we no longer consider this to
be Python version l.int.

- Recognize ``PYTHON: C:\PythonXY\python.exe`` in appveyor.yml.

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- Fix regression in updating tox.ini that looked like this::

envlist =
py27,py34,py35,py36

0.14.0 would incorrectly expand it to ::

envlist =
py27
py34
py35
py36

-------------------

- Improvements in Python version updating:

- preserve multiline ``python_requires=', '.join([...])`` expressions
(`GH 10 <https://github.com/mgedmin/check-python-versions/issues/10>`_)
- preserve generative envlists (``envlist = py{27,36}``) in tox.ini
(`GH 13 <https://github.com/mgedmin/check-python-versions/issues/13>`_)
- accept ``envlist=...`` with no spaces around the ``=`` in tox.ini
- preserve newline-separated envlists with no commas in tox.ini
- drop PyPy when dropping all supported Python 2.x versions
(`GH 11 <https://github.com/mgedmin/check-python-versions/issues/11>`_)

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- More robust Appveyor parsing: accept forward slashes (c:/pythonXY), do not
crash if the PYTHON environment variable doesn't point to a versioned Python
directory that we recognize (`GH 12
<https://github.com/mgedmin/check-python-versions/issues/12>`_).