Drop support for Python 3.9 (early 2026)

[srittau]

Python 3.9 is expected to reached its end-of-life phase around October 2025 (PEP 596).

How to remove support

EOL + 3 months (around January 2026)

• Update the documentation.
• Update the oldest_supported_python key in pyproject.toml.
• Remove requires_python >= 3.10 and python_version >= 3.10 from METADATA.toml
• Remove 3.9 from CI and test scripts.
• Update stubtest allow lists. Also check whether items can be moved to the common allow lists.

EOL + 6 months (around April 2026)

• Remove 3.9 exclusive branches from stubs.
• Remove obsolete stdlib modules (see VERSIONS).
  • _bootlocale
  • distutils.command.bdist_wininst
  • formatter
  • parser
  • symbol
• Replace typing_extensions imports.
  • ParamSpec
  • TypeAlias
  • TypeGuard
• Remove builtins.ellipsis after type checkers have switched to types.EllipsisType. (See Remove type checker-specific symbols from builtins.pyi and typing.pyi #7580)

(Please amend as necessary.)