Python 3.9 compability fixes with tests on GitHub Actions

[RyanMullins]

Fixes #8
Fixes #10

Makes the code compatible with Python 3.9.

• Converts | None type hints back to Optional (see PEP 604)
• Converts X | Y type hints to Union (see PEP 604)
• Removes use of match..case (see PEP 634, 635, 636)
• Removes kw_only= from dataclasses.dataclass() annotation.
• Uses the concrete import from synthid_text instead of the relative import from .
• Renames a helper function in the logits processor test to avoid it being picked up as a test case by PyTest