[Cleanup] Don't use django fork in build script

[c24t]

CI tests use a fork of django: https://github.com/c24t/django/tree/spanner/stable/2.2.x. This fork includes many changes required to run the django tests with spanner. See the full list here.

Decide how to handle this fork long term, and ensure that it's up to date with the most-recently released version of django (for each version we support). Check that we still need these patches after fast-forwarding the underlying django branch in #598.

Was previously:

The CI build script is using @timgraham's fork of django:

python-spanner-django/.kokoro/build.sh

Line 60 in 94ba284

mkdir -p $DJANGO_TESTS_DIR && git clone --depth 1 --single-branch --branch spanner-2.2.x https://github.com/timgraham/django.git $DJANGO_TESTS_DIR/django

It looks like this was added in 27222a3, but I can't tell why we needed to use this instead of django/django. Maybe because @timgraham merged a fix on his fork before it it was available on main?

In any case we should remove this now if we can. Related to the test cleanup work in #471.

[FirePing32]

@c24t Can I take this issue ?

[c24t]

@prakhargurunani if you're willing to give it a shot, definitely! @IlyaFaer may have more detail on the reasons we can't just use mainline django now.