a1c32702, 736cf54a, and 38787d0f remove uses of `simplejson` from
various parts of Swift in favor of the standard libary `json`
module (introduced in Python 2.6). This commit performs the remaining
`simplejson` to `json` replacements, removes two comments highlighting
quirks of simplejson with respect to Unicode, and removes the references
to it in setup documentation and requirements.txt.
There were a lot of places where we were importing json from
swift.common.utils, which is less intuitive than a direct `import json`,
so that replacement is made as well.
(And in two more tiny drive-bys, we add some pretty-indenting to an XML
fragment and use `super` rather than naming a base class explicitly.)
Change-Id: I769e88dda7f76ce15cf7ce930dc1874d24f9498a
1b8b08039a