python: be consistent with what value marks have in keywords

Marks are added to keywords in three places:

- `Node.add_marker`: name -> `Mark`
- `Function.__init__(callspec)`: name -> `Mark`
- `Function.__init__ iter_markers`: name -> True

I think it should be consistent, which will also help with some upcoming
code cleaning. The `Mark` seems more useful than just a `True`, so
switch to that.

src/_pytest/python.py

@@ -1675,7 +1675,7 @@ class Function(PyobjMixin, nodes.Item):
 
         self.keywords.update(
             {
-                mark.name: True
+                mark.name: mark
                 for mark in self.iter_markers()
                 if mark.name not in self.keywords
             }