Pin mypy to 1.11.2, avoiding updates that break CI (for now)

Separately, I have started a `mypy-1.12` branch to work on the broken
things. Meanwhile, though, we'd very much like for CI to keep working.

dev-requirements.txt

@@ -21,7 +21,8 @@ sphinx-rfcsection~=0.1.1
 # (or until py3.9 EOL... in 10/2025, I HOPE NOT)
 vcrpy @ git+https://github.com/sopel-irc/vcrpy@uncap-urllib3
 # type check
-mypy>=1.3,<2
+# often breaks CI on master, so pin and update deliberately, on our own terms
+mypy==1.11.2
 # for `pkg_resources`; first version in which it's typed
 # we don't use `pkg_resources` directly, but mypy still cares
 setuptools>=71.1