Support Named Unicode Characters in f-strings

[thatch]

Summary

This is a port of davidhalter/parso#160 plus a fix to yield expressions

Test Plan

A couple of new tests, but primarily tested in davidhalter/parso#160

[codecov-io]

Codecov Report

Merging #424 (94b6866) into master (16b30fe) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #424   +/-   ##
=======================================
  Coverage   94.31%   94.32%           
=======================================
  Files         232      232           
  Lines       22531    22532    +1     
=======================================
+ Hits        21251    21253    +2     
+ Misses       1280     1279    -1     

Impacted Files	Coverage Δ
libcst/_nodes/tests/test_atom.py	100.00% <ø> (ø)
libcst/_parser/conversions/expression.py	97.68% <ø> (ø)
libcst/_parser/parso/python/tokenize.py	83.18% <100.00%> (+0.02%)	⬆️
libcst/_typed_visitor.py	96.73% <0.00%> (+0.02%)	⬆️

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[thatch]

Upstream has merged their PR. Escape (shape) is only validated in f-strings for the time being.

[thatch]

Update based on testing thatch/python-grammar-changes@5cbc14e

                                                  23 24 25 26 27 30 31 32 33 34 35 36 37 38
examples/py36-0031-fstring-yield.py               .  .  .  .  .  .  .  .  .. .  .. oo oo oo 
examples/py36-0032-fstring-genexpr.py             .  .  .  .  .  .  .  .  .. .  .. oo oo oo 

[jimmylai]

LGTM, thanks for fixing this!