testing: add a file for checking no mypy errors

We probably something a bit more elaborate in the future but for now
it's something to verify fixes and catch regressions.

testing/typing_checks.py

@@ -0,0 +1,12 @@
+"""File for checking typing issues.
+
+This file is not executed, it is only checked by mypy to ensure that
+none of the code triggers any mypy errors.
+"""
+import pytest
+
+
+# Issue #7488.
+@pytest.mark.xfail(raises=RuntimeError)
+def check_mark_xfail_raises() -> None:
+    pass