Don’t quote characters twice.

Don’t quote characters twice.

``code`` markup is enough to mark command-line fragments or to talk
about a character.  ``'c'`` is still used for actual Python string
objects.  I did a similar change in optparse.rst in r86521.

I’ve also ported two minor changes from the 3.3 version of the file
(removing an unnecessary module name in a class directive, adding a
comma).