Drop support for Python 3.8 (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. (Officially drop Python 3.8 support #13386)
• Update the oldest_supported_python key in pyproject.toml. (Officially drop Python 3.8 support #13386)
• Remove requires_python >= 3.9 and python_version >= 3.9 from METADATA.toml (Officially drop Python 3.8 support #13386)
• Remove 3.8 from CI and test scripts. (Remove Python 3.8 from CI #13387)
• Update stubtest allow lists. Also check whether items can be moved to the common allow lists. (Remove Python3.8 stubtest allowlists #13389)

April 2025

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

(Please amend as necessary.)