chore(deps): update dependency pytest-django to v4 (GoogleCloudPlatform#4862)

* chore(deps): update dependency pytest-django to v4

* fix test failures for python2

Co-authored-by: Leah Cole <[email protected]>

Author: renovate-bot

appengine/flexible/django_cloudsql/requirements-test.txt

@@ -1,2 +1,2 @@
 pytest==6.1.1
-pytest-django==3.10.0
+pytest-django==4.0.0

appengine/standard/django/requirements-test.txt

@@ -1,2 +1,2 @@
 pytest==4.6.11; python_version < '3.0'
-pytest-django==3.10.0
+pytest-django==3.10.0; python_version < '3.0'

appengine/standard_python3/django/requirements-test.txt

@@ -1,2 +1,2 @@
 pytest==6.1.1
-pytest-django==3.10.0
+pytest-django==4.0.0