[Csv] _csv bug

[Skip Montanaro]

    Andrew> But this is broken:

    Andrew> [quotechar=None && p.quoting==1]

    Andrew> The obvious fix is to add an additional test to Parser_setattr
    Andrew> to disallow this combination. I've added this:
    ...
    Andrew> But I don't entirely like the idea of raising such a generic
    Andrew> error. If anyone has a better suggestion, let me know.

We never really decided on the split between sanity checkes in csv.py
vs. sanity checks in _csv.c did we?  I've got a change to csv.py ready to
check in which adds __init__ and _validate methods to the Dialect class.  If
we do more elaborate checks there, I think we can get away with coarser
checks and exceptions in _csv.c, bascially just stuff to keep the
interpreter from dumping core.

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