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>

tests/test_cli.py

@@ -6072,11 +6072,10 @@ class TestHttpx2LogRedaction(unittest.TestCase):
 class TestHttpx2UrlClassImport(unittest.TestCase):
     """GHSA-hjr3-rx4c-24cv: ``_HTTP_URL_CLASSES`` must recognize
     ``httpx2.URL`` whenever httpx2 is importable, not only when dnspython
-    imported it. dnspython 2.9 requires Python >= 3.11, so on Python 3.10
-    CI installs dnspython 2.8 (which never imports httpx2) while httpx2
-    still installs there (hatch in the ``[build]`` extra requires it), so
-    a constant taken from ``dns.query`` left ``httpx2.URL`` arguments on
-    the ``httpx2`` logger unmasked there."""
+    imported it. dnspython 2.8 never imports httpx2 while httpx2 can
+    still be installed (hatch in the ``[build]`` extra requires it), so a
+    constant taken from ``dns.query`` would leave ``httpx2.URL`` arguments
+    on the ``httpx2`` logger unmasked there."""
 
     def _exec_fresh_cli(self):
         """Execute parsedmarc.cli's source into a throwaway module and