chore(deps): update dependency django to v3.1.2 (GoogleCloudPlatform#4783)

Co-authored-by: gcf-merge-on-green[bot] <60162190+gcf-merge-on-green[bot]@users.noreply.github.com>

Author: renovate-bot

appengine/standard_python3/django/requirements.txt

@@ -1,2 +1,2 @@
-Django==3.1.1
+Django==3.1.2
 PyMySQL==0.10.1