Fix running automated tests on Python 3.6

This prevents the PyCapsule_GetPointer error.

plugins/UM3NetworkPrinting/src/__init__.py

@@ -0,0 +1,9 @@
+# Copyright (c) 2019 Ultimaker B.V.
+# Cura is released under the terms of the LGPLv3 or higher.
+
+# Workaround for a race condition on certain systems where there
+# is a race condition between Arcus and PyQt. Importing Arcus
+# first seems to prevent Sip from going into a state where it
+# tries to create PyQt objects on a non-main thread.
+import Arcus #@UnusedImport
+import Savitar #@UnusedImport