Warn when --reuse-db is used with an in-memory sqlite database

[ChrisJr404]

Closes #160.

Using --reuse-db with an in-memory sqlite database is a no-op: the database only lives for the duration of the process, so there's nothing to reuse or keep between runs. This trips people up (the default sqlite test database is in-memory), which is what the issue describes, and both Andreas and Daniel agreed a warning would be the right nudge.

So this emits a PytestWarning from django_db_setup when --reuse-db is requested (and --create-db isn't) and any of the databases being set up is an in-memory sqlite one. It only fires when you actually pass --reuse-db, so default runs are unaffected. Uses the same vendor == "sqlite" and is_in_memory_db() check already used for the live server.

Added tests for both the warning firing and staying quiet without the flag, plus a changelog line and a short note in the database docs.

[ChrisJr404]

Renamed the variable to in_memory_aliases as suggested.

On the repr: yes, the aliases are always strings, so it will not fail. I kept repr deliberately so the alias names come out quoted in the warning (e.g. 'default'), which reads more clearly than bare names when several are listed. Happy to drop it to a plain join if you'd prefer the unquoted form.