Remove Python 3.8 and 3.9 compatibility

[hakonanes]

Description of the change

Core dependency orix dropped Python 3.8 and 3.9 compatibility in v0.13.1, so it's fine for us to do the same.

This also means we can use types like list, tuple, dict etc. for type hints and replace Union[X, Y] with X | Y and Optional[int] with int | None etc. Makes reading and writing types much easier.

Progress of the PR

• [n/a] Docstrings for all functions
• [n/a] Unit tests with pytest for all lines
• Clean code style by running black via pre-commit

For reviewers

• The PR title is short, concise, and will make sense 1 year later.
• New functions are imported in corresponding __init__.py.
• New features, API changes, and deprecations are mentioned in the unreleased
  section in CHANGELOG.rst.
• New contributors are added to kikuchipy/__init__.py and .zenodo.json.