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