[Csv] _csv bug

[Skip Montanaro]

    Andrew> I'd like the underlying _csv module to be sane in it's own right
    Andrew> - I'd really rather these tests were kept in _csv. 

No argument here.  I'm just thinking that the _csv module only has to defend
against rotten inputs.  It can raise a generic error as far as I'm
concerned.

    Andrew> I also suspect we can move more functionalty from csv.py into
    Andrew> _csv to reduce overhead further. Some benchmarking is required -
    Andrew> it might be that we can become significantly faster by _csv talk
    Andrew> directly to fileobj when writing, etc.

What I'm talking about happens once, at Dialect instantiation time, so I
doubt performance is going to be a big issue.  It's also easier to give more
comprehensive feedback in Python.

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