[pkg] add python3 only pypi specifiers
This is only for pypi statistics purposes.
Also add 'Programming Language :: Python', looks obvious but seems to be sane to have this as well.
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Changelog history
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.. include:: 3.27.rst
.. include:: 3.26.rst
.. include:: 3.25.rst
.. include:: 3.24.rst
.. include:: 3.23.rst
.. include:: 3.22.rst
.. include:: 3.21.rst
.. include:: 3.20.rst
.. include:: 3.19.rst
.. include:: 3.18.rst
.. include:: 3.17.rst
.. include:: 3.16.rst
.. include:: 3.15.rst
.. include:: 3.14.rst