Compare strings properly in setup.py - fixes #14

[pzmarzly]

Comparing strings using is is like comparing char pointers in C - may work for constant string literals, but usually returns false. More info: https://stackoverflow.com/questions/1504717/why-does-comparing-strings-in-python-using-either-or-is-sometimes-produce

[pzmarzly]

$ python3
>>> from platform import system
>>> system() == 'Linux'
True
>>> system() is 'Linux'
False

[pzmarzly]

Merged in https://github.com/rmst/yoke/commit/a2c6cd3b681604b64a3a8c84656b3dc61b900b37

[rmst]

Ah damn, only saw this now. Should have merged this instead.