Remove deprecated python setup.py test call and update packager guide

[adferrand]

Fixes #7585

This PR removes the specific configuration to configure the test runner included in setuptools to use pytest, the deprecated parameters related to setuptools testing in setup.py, and update the packaging guide to use python -m pytest instead of python setup.py test.

The farm test test_sdist.sh is also updated to use directly pytest. This test is designed to reproduce the steps used by OS integrators when they package certbot, and ensure that we are not breaking something that will impact their work. We discussed with integrators from RHEL/CentOS and Debian, and they are fine with us testing sdist directly with pytest.

One execution of the test_sdist.sh farm test with the modifications made by this PR can be seen here: https://dev.azure.com/certbot/certbot/_build/results?buildId=2606&view=results

[bmw]

Thanks Adrien!

The only change I'd like to see is I think we should add a changelog entry about this.

I also wanted to double check that Fedora/EPEL is on board with this change. The message I saw talked about tox (and poetry), but not pytest. Unless you had a conversation with Felix that I was not a part of, I think we should tag them here just to make sure this will work for them.

[adferrand]

To add a note on the relevant PR, @bmw and I received an email yesterday from Felix (Fedora/EPEL integrator) saying he is fine with us using pytest directly.

[bmw]

Thanks for double checking that and keeping this thread updated!

All I think we need now is a changelog entry to help flag this change for packagers.

[adferrand]

Changelog added.

[bmw]

I almost got bit by #8272 again!

Can you merge master into this branch and add this to the 1.9.0 section?

Otherwise, this PR LGTM!