Fixed #23893: Added tzinfo to constant datetime in unit test

Added tzinfo to y2k constant (01/01/2000) in
custom_lookups.tests.DateTimeLookupTests.test_datetime_output_field
to fix warning message regarding naive datetimes.

Using default timezone from config (since pytz may not be available).
Thanks again. I prefer use at least one qualifier to avoid the (very small) chance of a name collision.