python: allow alternative middleware (observed [on LGTM](https://lgtm.com/projects/g/mozilla/mozillians/snapshot/9d6a7ee180addf7652fce96c21bafbad14d1dda7/files/mozillians/settings.py?sort=name&dir=ASC&mode=heatmap#L96))
python: allow alternative middleware
(observed [on LGTM](https://lgtm.com/projects/g/mozilla/mozillians/snapshot/9d6a7ee180addf7652fce96c21bafbad14d1dda7/files/mozillians/settings.py?sort=name&dir=ASC&mode=heatmap#L96))

github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql