Drop support for Python 3.7

[Pliner]

Summary

Python 3.7 EOL is approaching, this PR removes its support.

Related to #727.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Pliner]

At the moment, this test works for 3.8 as well.

[tomchristie]

Cool, thanks.

Dug out our discussion on dropping 3.6 to see what factors we used to help our decision here, which reminded me to check out pypi stats...

[T-256]

Python 3.7 reached EOL, but packages still can support it (it just not accepts new security fixes).
IMO would be better going to deprecation process before direct drop it.
I think <2% of downloads would be good point to complete drop.
According to pypyistats there is still 11% of downloads for py3.7:

[Pliner]

@tomchristie @T-256 Then, how is it better to proceed?

To create one more PR with a deprecation warning, release a new version, and wait for the updated statistics for a few months? Or just wait?

[tomchristie]

3.7 isn't really costing us anything at the moment.

Perhaps we wait until 3.12 is imminent and then consider switching to supporting 3.8, 3.9, 3.10, 3.11, 3.12?

[zanieb]

Other pull requests in the space:

• MAINT: Drop Python3.7 from supported versions. numpy/numpy#19665 (dropped in 2021!)
• drop python 3.7 support, drop pypy3.7-3.8, add pypy3.10 (except on windows) python-trio/trio#2668
• Add future warning for Python 3.7 EOL PrefectHQ/prefect#9469
• Drop support for Python 3.7 uvicorn#1996
• Drop support for Python 3.7 starlette#2178
• Drop support for Python 3.7 jazzband/pip-tools#1879
• Drop Python 3.7 pypa/pipx#1005
• Drop support for EOL Python 3.7 pypa/pip#11944 (good discussion here)

[tomchristie]

This is starting to impact our CI as other tools switch out... https://github.com/encode/httpx/pulls

Perhaps our next release really should be a 0.25 with Python 3.8+ support?

(Thanks for your patience @Pliner)

[Pliner]

This is starting to impact our CI as other tools switch out... https://github.com/encode/httpx/pulls

Perhaps our next release really should be a 0.25 with Python 3.8+ support?

(Thanks for your patience @Pliner)

+1.

Give me a moment, I will be rebase the PR on top of the latest changes.

[Pliner]

@tomchristie Sorry, I incorrectly rebased this PR from the first attempt and GH closed it :(

#2813