Winkerberos

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- Add Python 3.13 wheels.

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- Add support for Python 3.13.

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- Drop support for Python version 3.7.
- Add Secure Software Development Life Cycle automation to release process.
GitHub Releases now include a Software Bill of Materials, and signature
files corresponding to the distribution files released on PyPI.

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- Add support for Python 3.12. Drop support for Python versions 2.7, 3.5, and 3.6.

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- Add support for Python 3.11.

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- Fix build in mingw-w64: ``MINGW_CHOST`` is specific to msys2 build and
packaging environment. Instead, check ``sys.version`` which is available in
any python env. Also remove ``SecPkgContext_Bindings`` definition which has
been added in ``mingw-w64`` headers.
- Allow ``channel_bindings=None`` for ``authGSSClientStep``.
- WinKerberos now requires Python 2.7 or 3.5+.