Added python packages for things that were in vendor

- [ ] Once https://github.com/mozilla/zamboni-lib/pull/9 is merged, update vendor to the new commit.
It's also [used here](https://github.com/cvan/zamboni/blob/rm-amo-tasks/mkt/purchase/models.py?pr=%2Fmozilla%2Fzamboni%2Fpull%2F2220#L8)