Support Python 3.6 and 3.7
User story: I am a Liberty developer and administrator. I need ldh_middleware to run on both PureOS and Debian stable, so that I can both develop and administer it.
Suggested solution: Update Django to a version that supports Python 3.6 (Debian Stretch) and 3.7 (PureOS).
Next steps:
- Identify Django version that supports both 3.6 and 3.7.
- Confirm compatibility of ldh_middleware dependencies.
- Attempt rough upgrade (run site on new dependencies).
- Fix low-effort issues.
- Abort task if large-effort issues are found.