Revert refactor of old-style to new-style classes

As discussed in the mailing list, unfortunately this might break APIs
due to the subtle differences between new and old-style classes (see #2398).

This reverts commit d4afa1554b from PR #2179.

_pytest/assertion/__init__.py

@@ -57,7 +57,7 @@ class DummyRewriteHook(object):
         pass
 
 
-class AssertionState(object):
+class AssertionState:
     """State for the assertion plugin."""
 
     def __init__(self, config, mode):