Fixes https://github.com/certbot/certbot/issues/7913.
I only added the deprecation warning to `certbot.tests.util` because that's the only place where I think someone could be using the `mock` module through our API.
* remove external mock from acme
* update Certbot's mock usage
* remove mock dependency in plugins
* remove external mock from compatibility test
* add changelog entry
* Remove unsupported pylint disable options
* star-args removed in Pylint 1.4.3
* abstract-class-little-used removed in Pylint 1.4.3
* Fixes new lint errors
* Copy dummy-variable-rgx expression to new ignored-argument-names expression to ignore unused funtion arguments
* Notable changes
* Refactor to satisfy Pylint no-else-return warning
* Fix Pylint inconsistent-return-statements warning
* Refactor to satisfy consider-iterating-dictionary
* Remove methods with only super call to satisfy useless-super-delegation
* Refactor too-many-nested-statements where possible
* Suppress type checked errors where member is dynamically added (notably derived from josepy.JSONObjectWithFields)
* Remove None default of func parameter for ExitHandler and ErrorHandler
Resolves#5973
