Check-python-versions

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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>`_).

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- When updating a tox.ini keep multiline lists on multiple lines.

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- Support Python 3.8.

- Stop adding ``dist: xenial`` to .travis.yml as that is now the default.

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- Improvements in Python version updating:

- preserve comma style in python_requires lines
- no longer upgrade 'pypy' to 'pypy2.7-6.0.0' and 'pypy3' to 'pypy3.5-6.0.0'
because xenial now has 'pypy' and 'pypy3' available

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- Ignore unreleased Python versions (3.8 at the moment).

- Allow half-open ranges like ``--expect 3.5-``.

- Add experimental support for updating supported Python versions in
setup.py, tox.ini, .travis.yml, appveyor.yml and .manylinux-install.sh:

- command-line options --add and --drop to add and/or drop specific versions

- command-line option --update to explicitly enumerate all supported versions

- all changes are shown as diffs with confirmation before applying

- command-line option --diff to show the diffs and exit without any prompting

- command-line option --dry-run to re-run the parser and checker on in-memory
copies of updated files, to see if the update would succeed

- command-line option --only to limit the checks/update to some of the
supported files

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- Implement a full PEP-440 parser for python_requires.