assert that the root() is always a Module

The nodes are often created in an ad-hoc way, and their parent is not
   always set. We can't control for that invariant fully in the
   constructor, since the parent is sometimes set outside of the
   constructor. The previous commits did their best to clean up such
   situations, but let's add an assert just in case.

astroid/nodes/node_ng.py

@@ -23,7 +23,7 @@
     overload,
 )
 
-from astroid import util
+from astroid import nodes, util
 from astroid.context import InferenceContext
 from astroid.exceptions import (
     AstroidError,
@@ -333,11 +333,13 @@ def root(self) -> nodes.Module:
         :returns: The root node.
         """
         if not (parent := self.parent):
-            return self  # type: ignore[return-value] # Only 'Module' does not have a parent node.
+            assert isinstance(self, nodes.Module)
+            return self
 
         while parent.parent:
             parent = parent.parent
-        return parent  # type: ignore[return-value] # Only 'Module' does not have a parent node.
+        assert isinstance(parent, nodes.Module)
+        return parent
 
     def child_sequence(self, child):
         """Search for the sequence that contains this child.