Add typing for MachineNodeMap's constructor

Contributes to issue CURA-6793.

Co-Authored-By: Jaime van Kessel <nallath@gmail.com>

cura/Machines/ContainerTree.py

@@ -83,7 +83,7 @@ class ContainerTree:
     #
     #   This handles the lazy loading of MachineNodes.
     class MachineNodeMap:
-        def __init__(self):
+        def __init__(self) -> None:
             self._machines = {}
 
         ##  Returns whether a printer with a certain definition ID exists. This