Django-log-request-id

* Removes `requests.Session` passthrough functionality, see 58 for discussion.

* Adds support for async functionality in Django 3.0+.
* Adds support for Django 3.0+ to test matrix
* Drops support for unsupported Django versions

* You can now use the `LOG_USER_ATTRIBUTE` setting to control which attribute of the User model is logged.

- Adds the ability to change the default value of missing request_id (40)

- Fix encoding issue when installing package (38)

- Allow customization of log message (https://github.com/dabapps/django-log-request-id/pull/30)
- Make top-level scripts executable (https://github.com/dabapps/django-log-request-id/pull/19)
- Minor clarifications regarding setting request header (https://github.com/dabapps/django-log-request-id/pull/27)
- Change package to require Django `1.8`, and run tests against Django 2.2 (https://github.com/dabapps/django-log-request-id/pull/33)
- Add README to PyPI description (https://github.com/dabapps/django-log-request-id/pull/35)