Replace get_storage_class with new storages API for Django >= 4.2

[adnathanail]

In response to issue #1187 I've created a helper function _get_storage_class which detects whether to use the old or new API depending on the current Django version.

I've updated the code, and tests, to use this for now. Then when Django 4.1 support is dropped, it can be removed in favour of directly using storages.create_storage.

I also updated the docs about custom remote storages, so people know to use the new API if they are using Django >= 4.2

[carltongibson]

This stops the error for me if I run the tests so...

python -Werror::PendingDeprecationWarning -m django test --settings=compressor.test_settings compressor

[carltongibson]

I wonder if we should be using storages[backend] here (and likely override_settings) 🤔

(Rather than creating a new instance...)

[adnathanail]

I think currently storages[backend] wouldn't have anything in it?

Are we saying that users should be declaring a backend with compressor.storage.CompressorFileStorage for us to use, so we don't keep creating a new instance?

[carltongibson]

Well, the whole storages thing is new so... 😜

I think if we were designing from scratch, we'd likely say something like, "If you declare a backend in STORAGES we'll use that, otherwise we'll automatically create one for you".

So we'd need a setting for the backend name, and then we could look for that on first access.

That's just a first thought. Open to suggestions.

[carltongibson]

I wonder if we should be using storages[backend] here (and likely override_settings) 🤔

(Rather than creating a new instance...)

Compare #1202 from @PetrDlouhy which does it this way 🤔