* tests.unit.compat.mock: Remove legacy compat code
This removes old Python 3.4 compatibility code that is no longer needed.
* Prefer unitest.mock by universally using compat.mock
`mock` is a backport of the `unittest.mock` module from the stdlib, and
there's no reason to use it on newer Python versions.
* Fix wrong usages of ACMEProtocolException.
* Add changelog fragment.
* Fix error handling when content could not be decoded.
* Make sure that content_json is a dict or None.
* Improve acme_inspect's ACMEProtocolException handling.
* Improve error handling.
* Add tests.
* Fix challenge error.
* Add challenges tests.
* Provide content if available.
* Add some order tests.
* Linting.