FIX: fix error for setup.py for Python 3

The subprocess command in Python 3 returns a bytes object.  If the
homebrew subprocess check returns a not-empty result, then setup crashes
trying to combine the bytes with the string constants with and error
like "TypeError: Can't mix strings and bytes in path components."
Merge pull request #744 from matthew-brett/py3-setup-fix

FIX: fix error for setup.py for Python 3