Introduced subclasses for to be removed in django version X warnings

[claudep]

No description provided.

[charettes]

Great initiative this should ease the master branch major version bumping procedure.

By the way, I know we have a documented procedure for release but is it also the case for major version bumping?

I know those changes had to be made all by hands and I could live with those classes living in django.core.exceptions but IMHO they belong to the django.utils.deprecation one. If you agree I propose to do the whole renaming myself so you don't have to go through all files again and I get to do a full review of the changes at the same time.

[claudep]

I'm fine with moving them to django.utils.deprecation. Feel free to do it.