fix: address matplotlib typing issues and remove unnecessary casts

[alexander-held]

Add matplotlib to the mypy dependencies and fix all typing-related issues with matplotlib. There is one new # type: ignore for which I found no better solution. A few cast instances are also no longer needed, so removing those. Additionally, set two more optional mypy configuration options on the way towards strict mode: enable type-arg and disable implicit-reexport.

Following this, the only remaining things flagged by mypy --strict are of kind warn_return_any.

* add matplotlib to mypy dependencies
* fix matplotlib typing issues
* enable type-arg and disable implicit_reexport for mypy
* remove superfluous instances of cast()

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d5e3a14) to head (ac9a136).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #561   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           22        22           
  Lines         2132      2132           
  Branches       300       300           
=========================================
  Hits          2132      2132           

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