Make Lark generic on transformer return type

[erezsh]

Fixes #1186

One regression: subclasses of Lark using open() etc. will see a regression. But arguably it is pretty unusual, and we're fixing the much more common use-case.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.06%. Comparing base (3421420) to head (487c6ac).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1591      +/-   ##
==========================================
+ Coverage   90.01%   90.06%   +0.04%     
==========================================
  Files          52       52              
  Lines        8054     8089      +35     
==========================================
+ Hits         7250     7285      +35     
  Misses        804      804              

Flag	Coverage Δ
unittests	90.06% <100.00%> (+0.04%)	⬆️

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[erezsh]

@MegaIng Any chance you can give it a quick look?

[MegaIng]

IMO we should exclude test_typing.py from the coverage check to make the output cleaner. I don't think we currently have a coverage config file.

While reading through the logs I noticed that coverage can't parse test_pattern_matching.py since it's running with 3.8 and pattern matching is only support in 3.10+. I think moving up the coverage run from 3.8 to 3.10 is reasonable, but I am not going to insist if you don't want to.

[MegaIng]

The implementation looks good, we should just exclude the file from coverage.

The regression with subclasses is annoying, but I agree that this should be rare. It's jut a limitation of python's typing system not having HKTs

[erezsh]

Looks ok?