Temporarily disable automatic type checks in CI

MyPy checks should be renabled once the types are fixed in
#545

.github/workflows/mypy.yml

@@ -1,9 +1,5 @@
 name: Mypy
-on:
-  push:
-    branches: [ main ]
-  pull_request:
-    branches: [ main ]
+on: workflow_dispatch
 
 jobs:
   mypy_py3: