There were two issues in the implementation of install_git_hooks that

prevented it from working on Python 3:

- an old-style (0777) octal literal was used instead of a new-style one

  (0o777): old-style literals are not supported on Python 3; and

- bytes and unicode were being mixed in os.path.join, which is not an

  issue in Python 2 but is on Python 3.
Since prepare-dev.py is intended to install dependencies, we can't

assume the presence of six, so we determine the correct string type

manually.

Ran prepare-dev.py under Python 2 and Python 3 and it created git

hooks successfully.