* Don't call core constants from nginx plugin
* Move constants.py to _internal/
* Move ENHANCEMENTS from now-internal constants to public plugins.enhancements
* Update display.enhancements.ask from its 2015 comment
`certbot-compatibility-test` is using code in `acme` that I proposed making private and not trivially importable in https://github.com/certbot/certbot/issues/5775.
To fix it, I switched to using Certbot's test utilities which I proposed keeping public to help with writing tests for plugins. When doing this I had to change the name of the key because `rsa1024_key.pem` doesn't exist in Certbot.
I also deleted the keys in `certbot-compatibility-test`'s testdata because because they are unused.
* 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
