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.
Merge pull request #3737 from mhall1/ticket_23893

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