[issue2291] Raise a Py3K warning for catching non-BaseException exceptions

[Guido van Rossum]

Guido van Rossum <guido at python.org> added the comment:

"except object:" will continue to be a no-op, if only for compatibility.

With -3 it will issue a warning (I just checked this in, from
issue2371).  With -3 -Werror it will be an error.

We still need patches to issue -3 warnings for:
- raising exceptions that don't derive from BaseException
- __getitem__ or __getslice__ on exception instances

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<http://bugs.python.org/issue2291>
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