Revert "Update Django to 3.1 as well as other dependencies"

This reverts commit b3c9dca024.

src/requirements/dev.txt

@@ -1,9 +1,9 @@
-django-debug-toolbar==3.2
+django-debug-toolbar==2.1
 # Testing requirements
-pycodestyle==2.6.*
-pyflakes==2.2.*
+pycodestyle==2.5.*
+pyflakes==2.1.*
 pep8-naming
-flake8==3.8.*
+flake8==3.7.*
 codecov
 coverage
 pytest-cov
@@ -11,12 +11,12 @@ pytest==6.*
 pytest-django==4.*
 isort
 pytest-rerunfailures==9.*
-pytest-mock==3.5.*
+pytest-mock==2.0.*
 responses
 potypo
 freezegun
 
 # Not really required, just nice to have
-pytest-xdist==2.2.*
+pytest-xdist==1.31.*
 pytest-cache
 pytest-sugar