Add Literal to typing import shim, change to mypy-friendly approach.

[obi1kenobi]

Without the sys.version_info check, this used to require a type: ignore due to the following problem:
https://twitter.com/PredragGruevski/status/1308085206285529089?s=20

Mypy tracking issue for the import error approach is here: python/mypy#1393

For our needs, sys.version_info is an acceptable solution for Python import shims.

[codecov]

Codecov Report

Merging #926 into main will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #926   +/-   ##
=======================================
  Coverage   95.04%   95.04%           
=======================================
  Files         109      109           
  Lines        8734     8734           
=======================================
  Hits         8301     8301           
  Misses        433      433           

Impacted Files	Coverage Δ
graphql_compiler/typedefs.py	100.00% <100.00%> (ø)

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