[Distutils] Making install a no-op

[Ethan Furman]

I have recently received a request to make installing enum34 a no-op on Python3.4 and later so that wheels, etc, don't have to worry about the Python version when dealing with Enum.

 From an enum34 point-of-view this makes sense since Enum is in the stdlib in 3.4+, and enum34 has no purpose -- but how?  Is it a simple matter of checking for the Python version and raising 
SystemExit if enum34 is not needed?

--
~Ethan~