Make `BaseValidationError` base static in Python < 3.11

[layday]

Unsure if this worked in MyPy but Pyright failed to resolve the base of BaseValidationError subclasses leading to hard to trace type errors in custom note attributes from failing to narrow Iterable and ClassValidationError unions.

[layday]

Because str.index exists and it's a function on AttributeValidationNote iso of the IterableValidationNote's str | int, Pyright would expand note.index to ((__sub: str, __start: SupportsIndex | None = ..., __end: SupportsIndex | None = ..., /) -> int) | int | str 🙈

[codecov-commenter]

Codecov Report

Merging #379 (356b82c) into main (116d25b) will decrease coverage by 1.42%. The diff coverage is 100.00%.

:exclamation: Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

@@            Coverage Diff             @@
##             main     #379      +/-   ##
==========================================
- Coverage   95.80%   94.38%   -1.42%     
==========================================
  Files          26       26              
  Lines        2121     2120       -1     
==========================================
- Hits         2032     2001      -31     
- Misses         89      119      +30     

Impacted Files	Coverage Δ
src/cattrs/_compat.py	95.17% <100.00%> (-0.90%)	:arrow_down:

... and 2 files with indirect coverage changes

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more

[Tinche]

Cool, thanks!