Drop Python 2 support (#8591)

Fixes #8389 #8584.

This PR makes the necessary modifications to officially drop Python 2 support in the Certbot project.

I did not remove the specific Python 2 compatibility branches that has been added in various places in the codebase, to reduce the size of this PR and this will be done in a future one

* Update classifiers and python_requires in setup.py

* Remove warnings about Python 2 deprecation

* Remove Azure jobs on Python 2.7

* Remove references to python 2 in documentation

* Pin dnspython to 2.1.0

* Update changelog

* Remove warning ignore

pytest.ini

@@ -4,8 +4,6 @@
 [pytest]
 # In general, all warnings are treated as errors. Here are the exceptions:
 #   1- decodestring: https://github.com/rthalley/dnspython/issues/338
-#   2- Python 2 deprecation: https://github.com/certbot/certbot/issues/8388
-#      (to be removed with Certbot 1.12.0 and its drop of Python 2 support)
 # Warnings being triggered by our plugins using deprecated features in
 # acme/certbot should be fixed by having our plugins no longer using the
 # deprecated code rather than adding them to the list of ignored warnings here.
@@ -16,4 +14,3 @@
 filterwarnings =
     error
     ignore:decodestring:DeprecationWarning
-    ignore:Python 2 support will be dropped:PendingDeprecationWarning