Add support for Python 3.10 and test 3.11-dev

[hugovk]

Python 3.10 was released on 2021-10-04:

• https://discuss.python.org/t/python-3-10-0-is-now-available/10955

Let's add the Trove classifier and change tests from 3.10-dev to 3.10.

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Python 3.11.0 beta 1 has been released 🚀

This means it's now time for projects to start testing 3.11:

We strongly encourage maintainers of third-party Python projects to test with 3.11 during the beta phase and report issues found to the Python bug tracker as soon as possible. While the release is planned to be feature complete entering the beta phase, it is possible that features may be modified or, in rare cases, deleted up until the start of the release candidate phase (Monday, 2021-08-02). Our goal is have no ABI changes after beta 4 and as few code changes as possible after 3.11.0rc1, the first release candidate. To achieve that, it will be extremely important to get as much exposure for 3.11 as possible during the beta phase.

On GitHub Actions, 3.11-dev tracks the latest available alpha/beta/RC.

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Also bump GitHub Actions versions and update pre-commit.

[hugovk]

Whilst we're here, this library support Python 2.7 and 3.4+.

2.7-3.6 are EOL, maybe it's time to drop them?

Here's the pip installs for apipkg from PyPI for April 2022:

category	percent	downloads
3.7	35.59%	158,050
3.8	28.51%	126,584
3.6	10.91%	48,438
3.9	10.54%	46,818
null	7.50%	33,310
2.7	3.92%	17,401
3.10	2.10%	9,309
3.5	0.79%	3,502
3.4	0.14%	614
3.3	0.00%	19
3.11	0.00%	13
Total		444,058

Source: pip install -U pypistats && pypistats python_minor apipkg --last-month

[RonnyPfannschmidt]

im fine with the drops

[RonnyPfannschmidt]

@hugovk leaving the drops for a followup