ci: add infrastructure for generating custom stubs

[redeboer]

From typings/README.md:

Generate stub files (.pyi files) for external libraries using stubgen. For instance:

stubgen -p tensorflow -o typings

Alternatively, use Pyright to generate stub files that > contain docstrings as well:

pyright --createstub tensorflow

[codecov]

Codecov Report

Merging #216 (af373b9) into master (1cd8a0a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #216   +/-   ##
=======================================
  Coverage   81.10%   81.10%           
=======================================
  Files          13       13           
  Lines         725      725           
  Branches       99       99           
=======================================
  Hits          588      588           
  Misses        106      106           
  Partials       31       31           

Flag	Coverage Δ
unittests	81.10% <ø> (ø)

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