Drop support for Python 3.8 (January and April 2025)

[srittau]

Python 3.8 has reached its end-of-life phase on 2024-10-07 (PEP 569).

How to remove support

January 2025

• [ ] Update the documentation.
• [ ] Update the oldest_supported_python key in pyproject.toml.
• [ ] Remove 3.8 from CI and test scripts.
• [ ] Update stubtest allow lists. Also check whether items can be moved to the common allow lists.

April 2025

• [ ] Remove 3.8 exclusive branches from stubs.
• [ ] Remove obsolete stdlib modules (see VERSIONS).
  • _dummy_thread
  • _dummy_threading
  • dummy_threading
• [x] Replace typing_extensions imports: no op for Python 3.8 -> 3.9

(Please amend as necessary.)