Issue #8627: Fix "XXX undetected error" from unchecked PyErr_WarnPy3k return.

This is just a quick fix:  if the warning is turned into an exception, the
exception simply gets ignored.

Misc/NEWS

@@ -13,7 +13,10 @@ Core and Builtins
 -----------------
 
 - Issue #8627: Remove bogus "Overriding __cmp__ blocks inheritance of
-  __hash__ in 3.x" warning.
+  __hash__ in 3.x" warning.  Also fix "XXX undetected error" that
+  arises from the "Overriding __eq__ blocks inheritance ..." warning
+  when turned into an exception: in this case the exception simply
+  gets ignored.
 
 - Issue #8748: Fix two issues with comparisons between complex and integer
   objects.  (1) The comparison could incorrectly return True in some cases

Objects/typeobject.c

@@ -3869,10 +3869,16 @@ inherit_slots(PyTypeObject *type, PyTypeObject *base)
                                 (base->tp_hash != PyObject_HashNotImplemented) &&
                                 !OVERRIDES_HASH(type)) {
                     if (OVERRIDES_EQ(type)) {
-                        PyErr_WarnPy3k("Overriding "
+                        if (PyErr_WarnPy3k("Overriding "
                                            "__eq__ blocks inheritance "
                                            "of __hash__ in 3.x",
-                          1);
+                                           1) < 0)
+                            /* XXX This isn't right.  If the warning is turned
+                               into an exception, we should be communicating
+                               the error back to the caller, but figuring out
+                               how to clean-up in that case is tricky.  See
+                               issue 8627 for more. */
+                            PyErr_Clear();
                     }
                 }
             }