compat: handle SyntaxError when importing simplejson

We officially support Python 2.6 to 3.3, but simplejson does not
support Python 3.1 or 3.2:

  https://github.com/simplejson/simplejson/issues/66

Importing simplejson on Python 3.2 results in a SyntaxError because
simplejson uses the u'...' syntax (the syntax was not supported in
Python 3.0 to 3.2).

Support for loading simplejson instead of the stdlib json module was
added by #710:

  https://github.com/kennethreitz/requests/pull/710

No mention was made of the lack of support for Python 3.2, but it was
mentioned that simplejson can be faster than the stdlib json module.