CI: add type checking (#10997)

* Set up type checking with mypy.

* Make mypy pass.

* Use list() instead of sorted().

plugins/lookup/github_app_access_token.py

@@ -82,7 +82,7 @@ except ImportError:
 HAS_PYTHON_JWT = False  # vs pyjwt
 if HAS_JWT and hasattr(jwt, 'JWT'):
     HAS_PYTHON_JWT = True
-    from jwt import jwk_from_pem, JWT
+    from jwt import jwk_from_pem, JWT  # type: ignore[attr-defined]
     jwt_instance = JWT()
 
 try: