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

certbot-nginx/setup.py

@@ -4,7 +4,6 @@ import sys
 from setuptools import __version__ as setuptools_version
 from setuptools import find_packages
 from setuptools import setup
-from setuptools.command.test import test as TestCommand
 
 version = '1.9.0.dev0'
 
@@ -28,22 +27,6 @@ elif 'bdist_wheel' in sys.argv[1:]:
 elif sys.version_info < (3,3):
     install_requires.append('mock')
 
-
-class PyTest(TestCommand):
-    user_options = []
-
-    def initialize_options(self):
-        TestCommand.initialize_options(self)
-        self.pytest_args = ''
-
-    def run_tests(self):
-        import shlex
-        # import here, cause outside the eggs aren't loaded
-        import pytest
-        errno = pytest.main(shlex.split(self.pytest_args))
-        sys.exit(errno)
-
-
 setup(
     name='certbot-nginx',
     version=version,
@@ -82,7 +65,4 @@ setup(
             'nginx = certbot_nginx._internal.configurator:NginxConfigurator',
         ],
     },
-    test_suite='certbot_nginx',
-    tests_require=["pytest"],
-    cmdclass={"test": PyTest},
 )