Event-tracking

Move from nose to pytest for testing.

Python3.8 compatibility

Changes in Travis:
- Removed `Django<22`, `Py2.7`, `Py3.6`.
- Added `Py3.8`.

Changes in Tox:
- Removed `Django<22`, `Py2.7`.
- Added `Django30`, `Py3.8`.

This release updates the requirements of this repo, in particular, jinja2. This upgrade will fix the security vulnerability in the old versions of jinja2.

- allow a wider range of pymongo versions, so as not to pin consuming libraries.