Don't gather dirs ending .py

[kit1980]

Summary

codemod.gather_files should not try to gather dirs those names ending with .py, only regular files
(the issue encountered in an actual codebase).

Test Plan

libcst/tests/test_e2e.py updated

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (9eab2f0) 90.93% compared to head (8b62f45) 90.93%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #994   +/-   ##
=======================================
  Coverage   90.93%   90.93%           
=======================================
  Files         254      254           
  Lines       25974    25976    +2     
=======================================
+ Hits        23619    23621    +2     
  Misses       2355     2355           

Files Changed	Coverage Δ
libcst/codemod/_cli.py	57.77% <ø> (ø)
libcst/tests/test_e2e.py	97.61% <100.00%> (+0.11%)	⬆️

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[kit1980]

The CI errors look unrelated:

Error: Cache folder path is retrieved for pip but doesn't exist on disk: /home/runner/.cache/pip

[zsol]

Thanks!