PL: Add stub file for top-level __init__.py

[shnizzedy]

Related to #1426 by @pcasl
Related to #1220 by @effigies

Description

This PR adds a stub file for the top-level __init__.py, including an __all__ to expose the types of all symbols imported or defined therein.

Screenshots

Without the stub file

With the stub file

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.44%. Comparing base (4ee2a49) to head (0e7908c).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1436      +/-   ##
==========================================
+ Coverage   95.24%   95.44%   +0.19%     
==========================================
  Files         209      209              
  Lines       29782    29782              
  Branches     3357     4472    +1115     
==========================================
+ Hits        28366    28424      +58     
+ Misses        958      925      -33     
+ Partials      458      433      -25     

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