Add official support for Django 5.0

[browniebroke]

Didn't find anything that needed fixing, but let me know if I missed anything.

[tbicr]

Hi, thanks for PR

I left small comment for tox changes.

I also see changes for null_distinct in django/db/backends/base.py

django/django@4.2...5.0#diff-f7637259acc1ecbb74910a7df2879cae5f3209d7a87ab0ad06dbd594b27f1831L112-L133 than nice to add, for example in django_zero_downtime_migrations/backends/postgres/schema.py:

    if django.VERSION[:2] >= (5, 0):
        sql_create_unique = MultiStatementSQL(
            PGShareUpdateExclusive(
                "CREATE UNIQUE INDEX CONCURRENTLY %(name)s ON %(table)s (%(columns)s)s%(nulls_distinct)s",
                disable_statement_timeout=True
            ),
            PGAccessExclusive("ALTER TABLE %(table)s ADD CONSTRAINT %(name)s UNIQUE USING INDEX %(name)s"),
        )
    else:
        sql_create_unique = MultiStatementSQL(
            PGShareUpdateExclusive("CREATE UNIQUE INDEX CONCURRENTLY %(name)s ON %(table)s (%(columns)s)",
                                   disable_statement_timeout=True),
            PGAccessExclusive("ALTER TABLE %(table)s ADD CONSTRAINT %(name)s UNIQUE USING INDEX %(name)s"),
        )

and

    if django.VERSION[:2] >= (5, 0):
        sql_create_unique_index = PGShareUpdateExclusive(
            "CREATE UNIQUE INDEX CONCURRENTLY %(name)s ON %(table)s "
            "(%(columns)s)%(include)s%(condition)s%(nulls_distinct)s",
            disable_statement_timeout=True
        )
    else:
        sql_create_unique_index = PGShareUpdateExclusive(
            "CREATE UNIQUE INDEX CONCURRENTLY %(name)s ON %(table)s (%(columns)s)%(condition)s",
            disable_statement_timeout=True
        )

and add null_distinct unit tests in tests/unit/test_schema.py

And also nice to add changes to CHANGES.md

[browniebroke]

Found the PRs where support for nulls distinct was added to Django:

• Fixed #34701 -- Added support for NULLS [NOT] DISTINCT on PostgreSQL 15+. django/django#17058
• Fixed #35002 -- Made UniqueConstraints with fields respect nulls_distinct. django/django#17567

I feel like I've tested only 1 of the 2 attributes, hoping to find some inspiration on how to test the other in these PRs.

[tbicr]

Great work, thanks