Removed checking for PY2 for converting to string.

After examination the line and files of importance are:
kivy/config.py: [L541](https://github.com/kivy/kivy/blob/master/kivy/config.py#L541), [L300-L321](https://github.com/kivy/kivy/blob/master/kivy/config.py#L300-L321)
stdlib/ConfigParser (python3.3): [set and get](https://docs.python.org/3.3/library/configparser.html#legacy-api-examples)
stdlib/ConfigParser (python2.7): [set](https://docs.python.org/2/library/configparser.html#ConfigParser.RawConfigParser.set), [get](https://docs.python.org/2/library/configparser.html#ConfigParser.RawConfigParser.get)

After discussion beginning [in #kivy irc](https://botbot.me/freenode/kivy/msg/13953171/) (bot cuts out at about 4:07, but I try to reiterate as much as possible) it was found that the set method implemented in kivy/config.py L300-314 doesn't convert values to strings in python3 as is required by the set function in the ConfigParser from the standard library.

The original py2 check was implemented to guard against converting values to string incorrectly in python3 but from the article referenced in the [original commit](https://github.com/kivy/kivy/commit/76e4229ab652a803f1ac4e8f2d11429fd7dd25fc) in the "Drop 2.5, 3.1 and 3.2" section it actually suggests that 2.7 overlaps enough with 3.3+ that the syntax here is the same.

I've tested myself on python3.3 with success and since the code before the compatability commit was the same without the check then I'm fairly confident this will work on python2.x.
> tshirtman 04:20:35 PM
> ah, if it's that i guess we can drop it, we only support 2.7<x<3.0 and 3.3+

With this comment I expect that this change is acceptable.

I've also extended this logic to the ```get``` though I haven't actually had a problem with it.

Closes #2122.