mypy: Exclude broken typeshed stubs for markdown.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

mypy.ini

@@ -30,3 +30,9 @@ ignore_missing_imports = True
 
 # Warn of unreachable or redundant code.
 warn_unreachable = True
+
+[mypy-markdown.*]
+# The typeshed stubs for markdown in mypy 0.790 are very broken;
+# ignore them for now.  (https://github.com/python/typeshed/pull/4758)
+follow_imports = skip
+follow_imports_for_stubs = True