Drop Python 3.10 support in 4.0.0

[laughingman7743]

Use case

Python 3.10 reaches end of life on 2026-10-31. PyAthena drops a Python version around its end of life (3.9 was dropped in #617 after its October 2025 EOL), and 4.0.0 is expected around that date, in October 2026 or later. Dropping 3.10 in the major release avoids another breaking change in a later 4.x minor release.

It also unblocks #852: pandas 3.0 requires Python 3.11 or later, so the pandas extra can require pandas>=3.0 only after 3.10 is dropped.

Proposed change

• pyproject.toml: requires-python = ">=3.11", remove the 3.10 classifier, the tomli dev dependency, and the python_version markers that only exist for 3.10; target-version = "py311"; remove py310 from the tox envlist and [gh-actions].
• CI: remove 3.10 from the Test workflow's Python versions.
• README.md and docs/introduction.md: list CPython 3.11–3.14.
• benchmarks/pyproject.toml (requires-python) and benchmarks/tests/test_packaging.py.
• Remove 3.10-only compatibility code:
  • the tomli fallback in scripts/check_license_headers.py (and in scripts/verify_release_tests.py if Run the full test matrix in the Release workflow before publishing #862 is merged first);
  • the datetime.fromisoformat fallback in pyathena/sqlalchemy/array.py (_decode_datetime), if Python 3.11's parser accepts every Athena timestamp text it handles today.
• Regenerate uv.lock.
• Release note: breaking change, Python 3.10 is no longer supported; users on 3.10 can stay on the 3.x maintenance branch.

Validation plan (if implementing)

• just lint and the offline script tests.
• uv lock resolves for Python 3.11–3.14.
• The Test workflow passes on every remaining version (dispatch).