No longer compatible with Django 2.2, but still claims compatibility

[nburlett]

The commit for Added hashing function support #152 added an import for MD5 and other functionality that is not present in Django 2.2 (It was added in 3.0 .

Either the import needs to be conditional based on django version, or the setup.py shouldn't claim to support Django 2.2

[mShan0]

Good point. Opened a PR for it.