Syntax error on Python 3.8 and 3.9

pyproject.toml says the project supports python 3.8 and up. However, `__init__.py` uses the union type expression syntax `Optional[str | bytes]` which was only added in Python 3.10.

Either the version requirement should be raised, or the syntax should be reverted to the old version, i.e. `Optional[Union[str, bytes]]`