fix: import Optional when properties are nullable or not required

[dbanty]

A slightly modified cherry pick from #180 to fix the issue in 0.5

[codecov]

Codecov Report

Merging #181 into main will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              main     #181      +/-   ##
===========================================
- Coverage   100.00%   99.92%   -0.08%     
===========================================
  Files           41       41              
  Lines         1276     1251      -25     
===========================================
- Hits          1276     1250      -26     
- Misses           0        1       +1     

Impacted Files	Coverage Δ
openapi_python_client/parser/properties.py	100.00% <100.00%> (ø)
openapi_python_client/__init__.py	99.35% <0.00%> (-0.65%)	⬇️
openapi_python_client/parser/errors.py	100.00% <0.00%> (ø)
openapi_python_client/parser/openapi.py	100.00% <0.00%> (ø)
openapi_python_client/parser/reference.py	100.00% <0.00%> (ø)
openapi_python_client/parser/responses.py	100.00% <0.00%> (ø)

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