This reverts commit 64041f0e6e7a773574d7cde978d16305200004ea.
lookup.tests.LookupTests.test_regex_non_string fails under Postgres.
We should also try to rewrite the test using an existing model.
The recent improvements should make shared_models obsolete.
This reverts commit 1059da8de675442e84381d6366c0be254681753e, reversing
changes made to 4fa7f3cdd9bcf50ec4c7f64a31c1dfc02c375f46.