Gathering exports in augmented assignment statements

[Kronuz]

Summary

Add support for gathering exports in __all__ augmented assignments, such as:

import os
__all__ = []
__all__ += ["os"]

Test Plan

Run existing tests

[codecov-io]

Codecov Report

Merging #426 (e4864d5) into master (8eee3cc) will decrease coverage by 0.00%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #426      +/-   ##
==========================================
- Coverage   94.33%   94.33%   -0.01%     
==========================================
  Files         232      232              
  Lines       22586    22598      +12     
==========================================
+ Hits        21307    21318      +11     
- Misses       1279     1280       +1     

Impacted Files	Coverage Δ
libcst/codemod/visitors/_gather_exports.py	96.34% <87.50%> (-0.96%)	⬇️
...bcst/codemod/visitors/tests/test_gather_exports.py	100.00% <100.00%> (ø)

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[jimmylai]

LGTM, use a matcher can keep the code simpler.