0.13.0.dev1

django 2.2 and 3.0 only. python 3.5 - 3.8

there is some code change #169 and documentation change #172 but otherwise this is business as usual in terms of firming up our compatibility story with django.

once testing/community feedback turns positive the next will likely be an rc1 (just to have an rc), and then assuming no additional code change, straight to 0.13.0.