Create new user account using FXA python client

This is for https://github.com/mozilla/marketplace-tests/issues/534
@AndreiH Will you please open an issue against https://github.com/mozilla/fxa-python-client as I requested earlier? If no issue is opened they will have no idea that this is something we need. If they respond that they do not intend on fixing it themselves then we should submit a PR to them for the change.