Upgrading Django

This article documents notes on the process for upgrading Zulip to new major versions of Django. Here are the steps:

• Carefully read the Django upstream changelog, and git grep to check if we're using anything deprecated or significantly modified and put them in an issue (and then starting working through them). Also, note any new features we might want to use after the upgrade, and open an issue listing them; example.
• Start submitting PRs to do any deprecation-type migrations that work on both the old and new version of Django. The goal here is to have the actual cutover commit be as small as possible, and to test as much of the changes for the migration as we can independently from the big cutover.
• Check the version support of the third-party Django packages we use (git grep django requirements/ to see a list), upgrade any as needed and file bugs upstream for any that lack support. Look into fixing said bugs.
• Look at the pieces of Django code that we've copied and then adapted, and confirm whether Django has any updates to the modified code we should apply. Partial list:
• CursorDebugWrapper, which we have a modified version of in zerver/lib/db.py. See the issue for contributing this upstream
• PasswordResetForm and any other forms we import from django.contrib.auth.forms in zerver/forms.py (which has all of our Django forms).
• Our AsyncDjangoHandler class has some code copied from the core Django handlers code; look at whether that code was changed in Django upstream.
• Our FilteredManagementUtility in manage.py, which forks the management command discovery code.
• zerver/management/commands/change_password.py is forked from the upstream changepassword.py.