feat!: Support Django 4.2

[ankiaga]

Breaking changes

• Removes support for Django 2.2
• Transaction handling in Django 4.2 is different than in Django 3.2:
  • In Django 3.2: When Django tries to manually start a transaction while the dbapi connection is in AutoCommit=True mode, the Spanner Django provider would not start a transaction.
  • In Django 4.2: When Django wants to manually start a transaction while the dbapi connection is in AutoCommit=True mode, the Spanner Django provider will start a transaction.

Release notes

1. Adds support for Django 4.2
2. Removes support for Django 2.2
3. Only Python versions 3.8 and higher are supported
4. Adds support for transactions in AutoCommit=True mode by default for V4.2. Transactions in AutoCommit=True mode are by default not supported in prior versions (i.e. V3.2).
   • To enable manual transactions in AutoCommit=True mode in V3.2, please set ALLOW_TRANSACTIONS_IN_AUTO_COMMIT=True your settings.py file.
   • To disable transactions in AutoCommit=True mode in V4.2, please set ALLOW_TRANSACTIONS_IN_AUTO_COMMIT=False in your settings.py file.

[jaz-la]

as commented in this draft:
#860 (comment)

Currently Django 5 is released, since you are already working on it, can this field be override with a system environment, and make it something like:
if django.VERSION[:2] == (4, 2) or os.getenv('USING_DJANGO_4', 'false') == 'true'

[ankiaga]

Sorry we cant do that as we are not sure how will Django 5 behave with this code.
Please feel free to create a fork and make changes

[grjones]

Can I second @jaz-la's request? If we can appropriately set expectations with the env car, what's the harm? I am currently using a fork that is hard to maintain with Django 5, and this PR includes everything that I have needed for Django 5 so far. Please?

Thanks again for all of your work.

[jaz-la]

thanks a lot for working on it, left a few comments.
I'm hoping that we can also have a way to bypass the version check and fall into Django 4 logic with Django 5 at our own risk.
Another suggestion is to add a summary in the PR description so new developers can understand the actual difference between django 3 vs 4 without digging into the code.

[jaz-la]

thank you for working on it, I have been wanting to upgrade my django version for a long time and this is the last library that blocks me from doing so.