Fix: Resolve Python 3.9 compatibility issues, expand CI test matrix

[aaronsteers]

This adds Python 3.9 and 3.11 back into the test matrix.

Findings:

• Python 3.11 worked without any issue.
• Python 3.9 was failing because Pydantic wants to parse/evaluate type hints for classes derived from pydantic.BaseModel.
  • I disabled the lint rule that dissallowed the legacy syntaxes Union[x | y] and Optional[x].
  • In any subclasses of BaseModel, I reverted to legacy-style syntax declarations, to appease the Pydantic eval statements.

[aaronsteers]

@bindipankhudi - Another FYI above; this resolves the Py3.9 compatibility issues. No action needed. Going forward, we'll get CI failures if Python 3.9 or Python 3.11 tests fail (in addition to Python 3.10).