Fixes for all of the plugins. Added a script to invoke mypy. (I'm stiiiick of .bat files. They are just broken.)

plugins/3MFReader/__init__.py

@@ -1,16 +1,16 @@
 # Copyright (c) 2015 Ultimaker B.V.
 # Cura is released under the terms of the AGPLv3 or higher.
+from typing import Dict
 
 from . import ThreeMFReader
 from . import ThreeMFWorkspaceReader
 from UM.i18n import i18nCatalog
-import UM.Platform
+from UM.Platform import Platform
 catalog = i18nCatalog("cura")
 
-
-def getMetaData():
+def getMetaData() -> Dict:
     # Workarround for osx not supporting double file extensions correclty.
-    if UM.Platform.isOSX():
+    if Platform.isOSX():
         workspace_extension = "3mf"
     else:
         workspace_extension = "curaproject.3mf"