Convert property and endpoint names to snake case in Python code

[acgray]

Great project! A few contributions in this PR and more to come from a similar - but much shoddier! - tool I had knocked out for a current project.

This adds conversion to snake_case for property and API operation names. I've added some regex to avoid awkward conversions like h_t_t_p__response.

[dbanty]

Awesome work, thank you! A couple tiny suggestions, the only semi-important change is about calculating python_name on properties once instead of as a property which runs every time. Will affect performance a bit.

My only other request is, if you feel comfortable, try to add a property which doesn't use snake_case in the FastAPI app used for end to end testing so we verify that it preserves the name of the param while making a more appropriate one for Python. If not, I can stick one in there at a later time.

[codecov]

Codecov Report

Merging #29 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #29   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         9    +1     
  Lines          532       553   +21     
=========================================
+ Hits           532       553   +21     

Impacted Files	Coverage Δ
openapi_python_client/__init__.py	100.00% <100.00%> (ø)
openapi_python_client/openapi_parser/openapi.py	100.00% <100.00%> (ø)
openapi_python_client/openapi_parser/properties.py	100.00% <100.00%> (ø)
openapi_python_client/openapi_parser/reference.py	100.00% <100.00%> (ø)
openapi_python_client/openapi_parser/responses.py	100.00% <100.00%> (ø)
openapi_python_client/utils.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5583d86...ec5c87a. Read the comment docs.