Rename class names in type forward references

[florimondmanca]

Adds support for renaming type hints to async classes that use a forward reference, i.e. xyz: "AsyncXYZ".

(Support for renaming when the type hint is a regular name, i.e. xyz: AsyncXYZ, was already added via #49.)

[codecov[bot]]

Codecov Report

Merging #53 into master will increase coverage by 0.22%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   96.55%   96.77%   +0.22%     
==========================================
  Files           4        4              
  Lines         174      186      +12     
  Branches       44       46       +2     
==========================================
+ Hits          168      180      +12     
  Misses          2        2              
  Partials        4        4

Impacted Files	Coverage Δ
src/unasync/__init__.py	96.73% <100%> (+0.22%)	:arrow_up:
unasync/__init__.py	96.73% <0%> (+0.22%)	:arrow_up:

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