"mypy Gitter" is dead; it is called "python/typing" (#15610)

Author: stdedos

.github/ISSUE_TEMPLATE/config.yml

@@ -2,6 +2,6 @@ contact_links:
   - about: "Please check the linked documentation page before filing new issues."
     name: "Common issues and solutions"
     url: "https://mypy.readthedocs.io/en/stable/common_issues.html"
-  - about: "Please ask and answer any questions on the mypy Gitter."
+  - about: "Please ask and answer any questions on the python/typing Gitter."
     name: "Questions or Chat"
     url: "https://gitter.im/python/typing"