Remove deprecated python setup.py test call and update packager guide (#8262)

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

* Remove setuptools deprecated features about testing

* Updating packaging guide

* Add changelog entry

tests/letstest/scripts/test_sdists.sh

@@ -40,7 +40,7 @@ for pkg in acme certbot $PLUGINS; do
     tar -xvf "$pkg-$VERSION.tar.gz"
     cd "$pkg-$VERSION"
     python setup.py build
-    python setup.py test
+    python -m pytest
     python setup.py install
     cd -
 done