Merge branch 'bump-Django' into 'master'

Bump Python requirement Django to 3.1.5

See merge request !1157

base/requirements.txt

 boto3==1.15.16
 cryptography>=3.1
-Django==3.1.4
+Django==3.1.5
 elasticsearch==6.8.1
 lxml==4.5.2
 psycopg2-binary==2.8.6