MNT: Update coverage config

[effigies]

Added global exclusions to target the following cases:

• Typing-specific code that will not be reached by runtime
• __repr__. We can test them if it matters, but it usually won't, so don't need to incentivize contributors to add pointless tests.
• Code unreachable in tests (if 0:, if __name__ == '__main__')
• Abstract methods (explicitly marked or using raise NotImplementedError)

This allows us to remove # pragma: no cover comments, which should prevent these cropping up when new chunks of code start with a copy-paste.

Also, I'm removing the exclusion of tests, following https://nedbatchelder.com/blog/202008/you_should_include_your_tests_in_coverage.html.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.32%. Comparing base (30a5f3f) to head (ee1c9c4).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1338      +/-   ##
==========================================
+ Coverage   92.20%   95.32%   +3.12%     
==========================================
  Files          98      207     +109     
  Lines       12398    29084   +16686     
  Branches     2556     4863    +2307     
==========================================
+ Hits        11431    27724   +16293     
- Misses        644      927     +283     
- Partials      323      433     +110     

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[effigies]

I think this should be uncontroversial. Will wait until tomorrow to merge in case anybody wants to look this over.