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

Python 3.10 is expected to reach its end-of-life phase in October 2026 ([PEP 619](https://peps.python.org/pep-0619/#lifespan)). 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.)