Add support for Python 3.13 and drop EOL 3.7

[hugovk]

The Python 3.13 release candidate is out now! 🚀

The Release Manager has issued a call to action:

We strongly encourage maintainers of third-party Python projects to prepare their projects for 3.13 compatibilities during this phase, and where necessary publish Python 3.13 wheels on PyPI to be ready for the final release of 3.13.0. Any binary wheels built against Python 3.13.0rc1 will work with future versions of Python 3.13. As always, report any issues to the Python bug tracker.

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Also:

• Drop support for Python 3.7, it was end-of-life on 2023-06-27.
• Don't install the mock backport for tests, the stdlib version is used.
• Don't create universal wheels, they're only needed for Python 2 support.
• Upgrade syntax for Python 3.8+ with pyupgrade.

[coveralls]

coverage: 90.097%. remained the same
when pulling e9003cf on hugovk:add-3.13
into c003c22 on requests:master.