Expose more compatibility constants and ensure StopBits is numerical

[puddly]

Expand the migration guide to show how to migrate things like PARITY_NONE to Parity.NONE. I've also exported a few top-level constants we were missing.

StopBits has been converted into a float enum so that equality checks keep working.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.69%. Comparing base (ff630dd) to head (bc06afe).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #78      +/-   ##
==========================================
- Coverage   90.73%   90.69%   -0.04%     
==========================================
  Files          22       22              
  Lines        3367     3375       +8     
==========================================
+ Hits         3055     3061       +6     
- Misses        312      314       +2     

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