Support string annotations for type aliases

[Kronuz]

Summary

Add support for string annotations in type aliases.

Test Plan

Run tests.

This will support things like:

from typing import Dict, Union
from video import VideoType
VideoDict = Dict[str, Union[str, int, "VideoType"]]

And get a properly referenced VideoType.

[jimmylai]

Other than the formatting error. LGTM.
The error can be fixed by running tox -e autofix.

[codecov-io]

Codecov Report

Merging #401 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #401   +/-   ##
=======================================
  Coverage   94.11%   94.11%           
=======================================
  Files         231      231           
  Lines       22410    22427   +17     
=======================================
+ Hits        21091    21108   +17     
  Misses       1319     1319           

Impacted Files	Coverage Δ
libcst/_parser/types/tests/test_config.py	100.00% <ø> (ø)
libcst/tests/test_type_enforce.py	96.66% <ø> (ø)
libcst/metadata/scope_provider.py	97.03% <100.00%> (+0.04%)	⬆️
libcst/metadata/tests/test_scope_provider.py	100.00% <100.00%> (ø)

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