ci: bump Python to 3.13 across build and docs workflows

Adopts upstream's 3.13 bump. Also renames the uv cache key from py311
to py313 so a 3.11-built .venv is not restored under the 3.13
interpreter, and updates .python-version so local uv runs match CI.

pyproject requires-python stays >=3.11; 3.13 satisfies it and the
existing uv.lock resolves unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.python-version

@@ -1 +1 @@
-3.11
+3.13