Pin to workable mypy version

MasterMind2k · MasterMind2k · commit ec01acfe1e57 · 2019-02-18T22:13:23.000+01:00
Included flask stubs have wrong type annotations for some functions.
diff --git a/requirements/dev-3.txt b/requirements/dev-3.txt
@@ -1,2 +1,2 @@
 -r dev.txt
-mypy
+mypy==0.660
