Drop Python 3.10 and add Python 3.15 to the supported versions and CI (#953)

Python 3.10 reaches end of life in October 2026. requires-python is now
>=3.11, the unused requires_python key under [build-system] (not read by
hatchling, and its <3.15 bound read as excluding 3.15) is removed, and
Python 3.15 joins the CI matrix, classifiers, and support tables. Docs,
AGENTS.md, and comments that named 3.10 as the floor are updated.

Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

.github/workflows/python-tests.yml

@@ -65,7 +65,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.10", "3.11", "3.12", "3.13", "3.14"]
+        python-version: ["3.11", "3.12", "3.13", "3.14", "3.15"]
 
     steps:
     - uses: actions/checkout@v7