Make Parity a string enum

[puddly]

For better compatibility with pyserial, I've made Parity a string enum matching the existing constants.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.77%. Comparing base (0d770f6) to head (4ba7ee3).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev      #72   +/-   ##
=======================================
  Coverage   90.77%   90.77%           
=======================================
  Files          21       21           
  Lines        3348     3350    +2     
=======================================
+ Hits         3039     3041    +2     
  Misses        309      309           

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