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.

This has the added benefit of including proxies defined by the OS X System Configuration framework and in the Windows registry, rather than only checking os.environ.