[Csv] update - csv.py & libcsv.tex

[Skip Montanaro]

Just checked in new versions of csv.py and libcsv.tex.  The former includes
a couple changes from previous notes ("rest" param is not "restkey",
DictWriter objects now have user-configurable "extrasaction" to deal with
case of dicts which have keys not in the known fieldnames).  I added text
regarding the DictReader and DictWriter classes and fixed a number of Latex
errors in libcsv.tex.

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