python: add back instance accessor to all python nodes, not just Function

Regressed in 062d91ab4 (pytest 7.0.0rc1 only).

Fix #9486.

src/_pytest/python.py

@@ -283,6 +283,16 @@ class PyobjMixin(nodes.Node):
         node = self.getparent(Class)
         return node.obj if node is not None else None
 
+    @property
+    def instance(self):
+        """Python instance object the function is bound to.
+
+        Returns None if not a test method, e.g. for a standalone test function,
+        a staticmethod, a class or a module.
+        """
+        node = self.getparent(Function)
+        return getattr(node.obj, "__self__", None) if node is not None else None
+
     @property
     def obj(self):
         """Underlying Python object."""
@@ -1693,15 +1703,6 @@ class Function(PyobjMixin, nodes.Item):
         """Underlying python 'function' object."""
         return getimfunc(self.obj)
 
-    @property
-    def instance(self):
-        """Python instance object the function is bound to.
-
-        Returns None if not a test method, e.g. for a standalone test function
-        or a staticmethod.
-        """
-        return getattr(self.obj, "__self__", None)
-
     def _getobj(self):
         assert self.parent is not None
         if isinstance(self.parent, Class):