Use binary strings when writing setup.py as its required by distutils.

Attempting to pip install pantsbuild.pants currently fails with "TypeError:
'package' must be a string (dot-separated), list, or tuple". Looking into the
issue this is because setup.py is written with unicode strings, which
distutils does not support. Here we convert strings to binary just before
writing out setup.py.

Testing Done:
I built sdist's before/after this change, and I was able to pip install pants
with this change.

Reviewed at https://rbcommons.com/s/twitter/r/240/