python: don't redundantly duplicate parent markers to own keywords

This does have a slight semantic change: in a node hierarchy parent ->
child, if parent has a marker applied, then child is constructed, then
`parent.themarker = "overridden"`, previously
`child.keywords['themarker']` would return `True`, now it returns
`"overridden"`. But that's actually what I would have expected so I see
it as more of a bugfix.

Author: bluetech

src/_pytest/python.py

@@ -1676,10 +1676,11 @@ def __init__(
         # todo: this is a hell of a hack
         # https://github.com/pytest-dev/pytest/issues/4569
 
+        # Take own_markers only; NodeKeywords handles parent traversal on its own.
         self.keywords.update(
             {
                 mark.name: mark
-                for mark in self.iter_markers()
+                for mark in self.own_markers
                 if mark.name not in self.keywords
             }
         )