🔧 add tox environment for mypy (#12376)

Author: chrisjsewell

tox.ini

@@ -62,3 +62,11 @@ extras =
 commands =
     ruff format .
     ruff check --fix .
+
+[testenv:mypy]
+description =
+    Run mypy type checking.
+extras =
+    lint
+    test
+commands = mypy {posargs}