ci: test Python 3.11 through 3.15

[vvillait88]

Summary

Tests the package on every Python it supports, and declares that set.

• A test job runs the suite on 3.11, 3.12, 3.13, 3.14 and 3.15 (3.15 is a release candidate until its October release). The existing job keeps lint, format, type, dead-code and coverage checks on 3.12.
• The classifiers name 3.11 through 3.15.
• Ruff targets py311, matching requires-python = ">=3.11". 3.10, which reaches end of life in October, was already outside the supported range.
• The new target's UP017 rule moves two timezone.utc uses to datetime.UTC, the same object.

Type of change

• Bug fix (no breaking change)
• New feature (no breaking change)
• Breaking change (existing callers must update)
• Docs, tests, or internal maintenance only

Public API

None.

Test plan

The suite was run locally on each interpreter with a separate environment per version: 1886 passed and 4 skipped on 3.11, 3.12, 3.13 and 3.14. On macOS, 3.15.0rc2 cannot import pydantic-core's published 3.15 wheel (the dynamic loader rejects the binary), which is outside this package; the Linux job in this PR's CI is the evidence for 3.15. ruff, format, ty (package and examples), vulture and the full suite on 3.12 pass locally.

Checklist

• Tests cover the new behavior, and the suite passes locally
• Lint, format, and type checks pass
• Docs and README examples updated if the public surface changed (no public surface changed)
• No secrets, credentials, or personal data in the diff or the tests