Rewrite on Pydantic

[shenanigansd]

Closes #103, rewriting on top of Pydantic instead of dataclasses to avoid blowing up on spec changes.

This PR also includes some QoL changes:

• Switching from the bespoke make.ps1 to Nox

[codecov]

Codecov Report

Attention: Patch coverage is 83.78378% with 6 lines in your changes missing coverage. Please review.

Project coverage is 76.04%. Comparing base (70ee6c1) to head (10d2f6e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/letsbuilda/pypi/async_client.py	0.00%	3 Missing ⚠️
src/letsbuilda/pypi/sync_client.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
- Coverage   77.35%   76.04%   -1.32%     
==========================================
  Files           8        8              
  Lines         212      167      -45     
==========================================
- Hits          164      127      -37     
+ Misses         48       40       -8     

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