tox: Bump and pin mypy to 1.13.0

django-stubs package requires mypy >= 1.12 and its latest release
pins mypy to >= 1.14.0 which fails to validate some valid typings
related to the standard Python module email.

So bump and pin mypy to 1.13.0 to avoid typing errors in Jenkins CI.

tox.ini

@@ -45,7 +45,7 @@ setenv = DJANGO_SETTINGS_MODULE = swh.web.settings.development
 extras =
   testing
 deps =
-  mypy==1.11.1
+  mypy==1.13.0
 commands =
   mypy swh