Remove entirely unnecessary and unused bits from requests.compat

Instead of only checking one or another type of string-like object that
we accept, let's be able to check both. Previously, we only checked if
the filename was an instance of the native str type which on Python 2
excluded unicode filenames and bytes-like filenames on Python 3.

Fixes #2411

I just fixed a minor typo: "throws" is misspelled as "thows".

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.