Django-qsessions

- Added support for Python 3.7, Django 2.1.
- Used pytest for testing.
- Improved session delete performance (reduce number of queries)
- Refactored codes

Thanks [akx](https://github.com/akx), [saeed617](https://github.com/saeed617)

- Improve docs

- Fixed a bug when User Agent is an empty string

- Fixed migrations for `created_at` field.