[Csv] why push so much code into C?

[Skip Montanaro]

I see that csv.py has dwindled down to next-to-nothing.  Even the dialect
registry stuff is in C.  (Is the Dialect class in csv.py used anymore?  I
see something which looks like a dialect object in the C code.)  It's not
obvious to me that there's any performance gain to be had by having anything
other than the raw parsing and writing code in the C module.  On the other
hand, by pushing code which isn't performance-critical into C it becomes
harder to maintain and extend, and significantly limits the number of people
who can contribute to the code's growth and maturity.

Skip