Merge branch 'django31' into 'master'

Add support for Django 3.1

See merge request !574
17 jobs for master in 20 minutes and 50 seconds (queued for 1 minute and 55 seconds)
Status Job ID Name Coverage
  Test
passed #982556800
bandit-sast

00:00:37

passed #982556829
coverage

00:07:31

46.0%
passed #982556839
django-latest

00:08:16

passed #982556835
git-heads

00:07:33

passed #982556833
pep8

00:02:14

passed #982556804
py36-django-20

00:06:27

passed #982556806
py36-django-21

00:06:44

passed #982556811
py36-django-22

00:06:32

passed #982556815
py36-django-30

00:07:11

passed #982556803
py37-django-20

00:06:35

passed #982556808
py37-django-21

00:06:36

passed #982556814
py37-django-22

00:07:09

passed #982556817
py37-django-30

00:07:21

passed #982556820
py38-django-22

00:06:04

passed #982556818
py38-django-30

00:06:45

passed #982556822
py38-django31

00:06:57

passed #982556825
py39-django31

00:07:17