Extend compatibility back to Python 3.8

[eemeli]

This is needed to be able to use the command for linting Firefox, as mach still supports 3.8.

Changes consist of:

• Using if..elif..elif..else instead of match (PEP 634)
• from __future__ import annotations everywhere (PEP 585)
• Using old type syntax for type aliases and cast targets (PEP 585)
• Using importlib_resources rather than using the built-in importlib.resources in tests (bpo-39791)
• The pyproject.toml fixes from 2d6439d