Type hint mixin base class, fix some more minor style and typing issues

[theOehrly]

Allows type checkers to detect the AutosummaryDocumenter as a mixin that has all the instance attributes of a any class that subclasses the Documenter base class. This makes the IDE go less crazy about accessing undefined variables and potentially allows stronger type checking/linting in the future.

Also, some more minor fixes.

WIP, maybe some more changes to come in the next few days. Just FYI for now.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 60.38%. Comparing base (6834615) to head (40871cc).
Report is 7 commits behind head on master.

Files	Patch %	Lines
autodocsumm/__init__.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
- Coverage   61.89%   60.38%   -1.51%     
==========================================
  Files           2        2              
  Lines         664      669       +5     
==========================================
- Hits          411      404       -7     
- Misses        253      265      +12     

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[theOehrly]

This can stay as it is, from my point of view. Nothing to add anymore.