Allow $schemas in codegen

[leipzig]

This addresses #345
$schemas should be accepted and added to the loadingOptions

[mr-c]

FYI @leipzig , you might benefit from make format after you pip install black (if needed)

[codecov]

Codecov Report

Merging #346 into main will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #346      +/-   ##
==========================================
+ Coverage   69.67%   69.74%   +0.07%     
==========================================
  Files          16       16              
  Lines        2846     2846              
  Branches      745      745              
==========================================
+ Hits         1983     1985       +2     
  Misses        677      677              
+ Partials      186      184       -2     

Impacted Files	Coverage Δ
schema_salad/python_codegen.py	98.55% <ø> (ø)
schema_salad/ref_resolver.py	84.25% <0.00%> (+0.14%)	⬆️
schema_salad/fetcher.py	71.73% <0.00%> (+1.08%)	⬆️

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[mr-c]

Thank you @leipzig !