Drop support for Python 3.10 (around January/April 2027)

[srittau]

Python 3.10 is expected to reach its end-of-life phase in October 2026 (PEP 619). We will remove support in two steps, approximately three and six months after it reached EOL.

How to remove support

Expected January 2027

• Update the version in README.md.
• Update various fields in pyproject.toml.
• Remove 3.10 from CI and test scripts.
• Update stubtest allow lists. Also check whether items can be moved to the common allow lists.
• Remove requires-python >= 3.11 and python-version >= 3.11 from METADATA.toml.
• Remove tomli from requirements-tests.txt.

Expected April 2027

• Remove 3.10 exclusive branches from stubs.
• Remove obsolete stdlib modules (see VERSIONS).
  • binhex
  • distutils.command.bdist_msi
• Replace typing_extensions imports.
  • TypeVarTuple
  • Required/NotRequired
  • Self
  • LiteralString
  • dataclass_transform

(Please amend as necessary.)