Fix for not parsing subscripts such as cast()["from"]

[Kronuz]

Summary

When handling subscripts, cast(), and potentially others such as TypeVar() and NewType(), were being treated as potential string type hints, resulting in snippets like the following failing with a ParserSyntaxError:

cast(Dict[str, str], {})["from"]

Test Plan

Run tests

[codecov-io]

Codecov Report

Merging #428 (fd444fe) into master (8eee3cc) will decrease coverage by 0.02%.
The diff coverage is 76.00%.

@@            Coverage Diff             @@
##           master     #428      +/-   ##
==========================================
- Coverage   94.33%   94.31%   -0.03%     
==========================================
  Files         232      232              
  Lines       22586    22606      +20     
==========================================
+ Hits        21307    21321      +14     
- Misses       1279     1285       +6     

Impacted Files	Coverage Δ
libcst/metadata/tests/test_scope_provider.py	99.20% <68.42%> (-0.80%)	⬇️
libcst/metadata/scope_provider.py	96.57% <100.00%> (+<0.01%)	⬆️

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[jimmylai]

LGTM.
@Kronuz thanks for fixing this quickly.