refactor(hugr-py): remove module underscores

[ss2165]

and other minor refactors to set up docstrings (pulled out from larger PR to make easier merges)

let's try and not do a release between this and the docstring PR merge

[codecov]

Codecov Report

Attention: Patch coverage is 86.28571% with 24 lines in your changes missing coverage. Please review.

Project coverage is 86.99%. Comparing base (c21753d) to head (24319a7).

Files	Patch %	Lines
hugr-py/src/hugr/serialization/tys.py	75.92%	13 Missing ⚠️
hugr-py/src/hugr/serialization/ops.py	90.90%	7 Missing ⚠️
hugr-py/src/hugr/dfg.py	84.61%	2 Missing ⚠️
hugr-py/src/hugr/ops.py	83.33%	1 Missing ⚠️
hugr-py/src/hugr/val.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1230   +/-   ##
=======================================
  Coverage   86.98%   86.99%           
=======================================
  Files         100      100           
  Lines       19023    19021    -2     
  Branches    17040    17040           
=======================================
- Hits        16548    16547    -1     
+ Misses       1697     1696    -1     
  Partials      778      778           

Flag	Coverage Δ
python	91.46% <86.28%> (+0.04%)	⬆️

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