Replace usages of unipath package

[StephenChan]

From a comment in issue #182:

• unipath: 1.1 (last update 2015, should remove in favor of pathlib)
  • This appears to be a little messy to replace with pathlib while we're not on Python 3 / Django 2 yet. Many Django settings don't accept pathlib.Path objects at this point, so we'd need to convert back and forth between str and pathlib.Path in several places. Probably easiest to just keep this until we're on Python 3.

This could be taken care of now, although it could also be easier later when we're at least on Django 3.1, because several Django settings still didn't accept pathlib.Path objects until then. https://docs.djangoproject.com/en/dev/releases/3.1/