Remove support for Python 3.8

[snejus]

Description

• Drop support for EOL Python 3.8 making Python 3.9 the minimum supported version

• Take advantage of Python 3.9+ type hint syntax by:

  • Using list[T] instead of List[T] etc. from typing module
  • Using Type | None syntax for unions instead of Union[Type, None]
  • Moving collection type hints from typing to collections.abc
  • Using TYPE_CHECKING guard for runtime import optimization

Note: in #5503 we found that we cannot support Python 3.12 unless we upgrade our minimum support Python to 3.9.

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)