As of r61478, the following code fails to produce a py3k warning:

try:
   raise ValueError
except ((ValueError,object),):
   pass


which is an error for in py3k:

Traceback (most recent call last):
  File "x.py", line 3, in <module>
    except ((ValueError,object),):
TypeError: catching classes that do not inherit from BaseException is 
not allowed

Should nested tuples in an except clause be a valid syntax?
like:
...
except ((ValueError, Exception), TypeError, (...)):
    pass

I don't see any benefit and because of this behavior the implementation
(in Python/errors.c and in the patch attached) will use recursive function. 

I think that any objects inside a tuple in an except clause should be
treated equally (as an object), even a tuple, so the except clause above
should be an error in py3k and should produce a warning because the list
objects does not inherits BaseException.

Please, see my comment in http://bugs.python.org/issue2345, it is
related to this issue.

issue2380.diff does not apply anymore.

Not all incompatibilities have to raise a py3k warnings; AFAIK, only those which are not handled by 2to3 do.