chore!: drop Python 3.10, which reached end of life

[lesnik512]

Closes #174.

Python 3.10 reached end of life on 2026-10-01 (PEP 619). The floor is now 3.11.

Changes

• pyproject.toml: requires-python = ">=3.11,<4", and the 3.10 trove classifier is gone. ruff has no target-version of its own; it takes the target from requires-python, so it moves to py311 with no separate edit.
• CI (_checks.yml): 3.10 is out of the pytest and floors matrices. The lint and install-isolation jobs, which ran on the lowest supported interpreter, now run on 3.11. The same goes for security-audit.yml.
• instruments/base.py: typing_extensions.Self becomes typing.Self, which is in the stdlib from 3.11.
• The comments on the core dependency and the sentry markers now say 3.11 where they said 3.10.

Not changed

• typing-extensions stays a core dependency. HealthCheckTypedDict still needs typing_extensions.TypedDict, because pydantic rejects a stdlib TypedDict model below 3.12. ADR-0005 (the issue calls it ADR-0007) sets its revisit trigger at 3.12, and this PR doesn't reach it.
• The 3.13t free-threaded leg stays. Its extras exclusions are unchanged (fastmcp blocked on free-threaded Python 3.13 (cffi gates Py_GIL_DISABLED to 3.14+) #172, litestar blocked on free-threaded Python 3.13 (msgspec gates Py_GIL_DISABLED to 3.14+) #173).

Verification

Run locally:

• 3.11: just lint-ci passes (ruff and ty), and just test-ci passes 336 tests at 100% coverage.
• 3.14: the full suite passes 336 tests.

The floors job (lowest-direct resolution) and the install-isolation job on 3.11 run only in CI.