Create pycoverage extension

[Lonami]

Subject: introduce a more powerful alternative to the existing coverage extension.

Feature or Bugfix

• Feature

Purpose

• The existing coverage extension has certain limitations.
• For one, I couldn't really get it to work in my project out of the box.
• But, it also lacks the ability to check that all parameters and return types are documented.

Detail

• This is currently missing tests. But it seems to work on my project.
• There might be far better ways to achieve some of the things I'm trying to do (such as, better way to query existing fields, or access the parsed docstrings).
• I also would like for this to check that all docstrings contain code examples, and that they are syntactically valid.
• I didn't just update the original code because I felt I was changing too much and it also uses different file formatting. Maybe the current formatting here is wrong though. Maybe it's better to improve that extension.

Relates

• Fix coverage extension crash when module contents are not found  #11702 (comment)

[chrisjsewell]

Heya thanks, I feel though it might be better for you to first create this as a third-party extension, in your own repo.
Then you and others can play around with it more, and we could later assess if it warrants going into core