Check-python-versions

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- Treat versions like ``3.10.0-beta.3`` as prerelease versions. `GH 28
<https://github.com/mgedmin/check-python-versions/issues/28>`_.

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- PyPy support is now checked for consistency: if setup.py declares PyPy
support, you need to also run tests on PyPy in your primary CI systems (tox,
GitHub Actions, Travis CI), and vice versa. Secondary CI systems (Appveyor,
manylinux.sh) are excluded from this check. `GH 26
<https://github.com/mgedmin/check-python-versions/issues/26>`_.

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- Support ``pypy-N.M[-vX.Y.Z]`` in GitHub Actions, in addition to
``pypy2``/``pypy3`` (`issue 24
<https://github.com/mgedmin/check-python-versions/issues/24>`_).

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- Initial supprot for GitHub Actions (`issue 22
<https://github.com/mgedmin/check-python-versions/issues/22>`_).
- Correctly detect the end of a YAML list even if it's not indented.
- Try to preserve envlist order in ``tox.ini``.
- Preserve quotes around Python versions in Travis matrix jobs.

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- Preserve trailing commas in ``tox.ini``.
- Do not duplicate comments in ``tox.ini``.
- If all versions in .travis.yml are quoted, keep the quotes.

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

- Add support for Python 3.9.

- Preliminary Python 3.10 support: sort 3.10 after 3.9, quote "3.10" when
updating YAML files so it doesn't get treated as a floating point
number (`GH 19
<https://github.com/mgedmin/check-python-versions/issues/19>`_)