Remove pre-2.6 compatibility imports/aliases

also
* remove aliasing of `MutableMapping` to `DictMixin`
* remove duplicate `sys` import
* ~~move conditional urlopen and urlencode to top~~
* shuffle imports around to try and better match pep8

there's a few imports left in the module body (`os`, `tempfile`, `webbrowser` imported in `open_in_browser`), not sure I should fix them.

Also I *think* `__fix_docstring` could be altered to only munge the byte and unicode literals for versions of Python between 3.0 and 3.2 inclusive (since 2.6+ has byte literals, and 3.3 reintroduced unicode literals)