Add support for Python 3.13 and drop EOL 3.7

[hugovk]

The Python 3.13 release candidate is out now! :rocket:

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 e9003cffa5e1dd3a35317a499120dbbcfd9889b1 on hugovk:add-3.13 into c003c22eceb9e54a7cfe242b7976412a2c98a972 on requests:master.

[hugovk]

Closing this year-old PR due to lack of interest, but happy to re-open if that changes! Note Python 3.14 is almost out and also needs testing.