Some basic Python 3 fixes

Summary:
- Add 'six>=1.4.1' to dependencies (for six.moves.urllib.parse)
- Explicit relative imports
- Replace sys.maxint with 2**64-1 (maybe we should do something
  smarter?)
- Replace deprecated 'except' syntax with 'except...as'
- Mandatory parens around the argument to print()

Test Plan:
The Flask server now starts under Python 3.4.1.  I haven't tested it
beyond that yet.

Reviewers: charles

Reviewed By: charles

Differential Revision: https://review.inboxapp.com/D372