Drop Python 3.9 support

[maxrake]

Description

This is a placeholder issue to reference in source control. It is meant to capture all the changes that are needed when Python 3.9 support is no longer required.

Additional Details

• This issue could be completed to coincide with supporting Python 3.13, to stick with the "latest plus three previous releases" paradigm
• Consider using pyupgrade to automatically upgrade to 3.10+ syntax
  • Update the target-version key for the [tool.ruff] config in pyproject.toml
  • Use pyupgrade or ruff check --fix directly to make the changes to the files
• Update the python_version key value in the [tool.refurb] config in pyproject.toml
• Ensure any packages that are only required for Python 3.9 are removed
• Update idioms used for Python 3.9 support only
  • Look for TODO: comments with this issue referenced
  • Type annotations can use the pipe operator for unions (or X | None instead of Optional[X])
  • Parenthesized context managers
    • These started working in py3.9a6 but are not officially supported until py3.10
    • Ref: https://stackoverflow.com/q/67808977
  • Structural pattern matching
  • Look for more in "What's New in Python 3.10"
• Update the testing environments
  • in tox, GitHub workflows, etc.
  • Update the test_python_version test in tests/unit/test_package_metadata.py
• This might also be a good time to switch from black to ruff for formatting, if that change is desired

Acceptance Criteria

• Python 3.9 support is removed
• Documentation is updated