Add six to requirements.txt

Some modules import six, we can see some .py files contain
"import six", so add it to requirements.txt.

Change-Id: I74cd752256e581ce38aac75af9a6154fc8c62e73
Support building wheels (PEP-427)

With that, building and uploading wheels to PyPI is only one "python
setup.py bdist_wheel" away.

Change-Id: I93cee533132d59955b558da34c86ba5d310b2869
Merge "Support building wheels (PEP-427)"