Support for Django 4 (#101)

* Support for Django 4

* Change pytest-django version

* Change psycopg2 version

* Change pytest version

* Change tox version

* Change pytest-cov version

* Update circle ci jobs

* Fix ci jobs

* Update ci to postgres 12

* Allow Django 5.0

* Bump Django min version to 3.2

* Fix CI to account for >= 3.2 requirement

* Fix quote

* Remove python 3.12 for now due to distutils removal

* Remove Django upper bound

* Add back python3.6,3.7 wfs

* Downgrade dj-database-url

* Fix isnull issue from Django 4

tests/test_isnull.py

@@ -0,0 +1,51 @@
+import django
+import pytest
+
+from django.test import TestCase
+
+from localized_fields.fields import LocalizedField
+from localized_fields.value import LocalizedValue
+
+from .fake_model import get_fake_model
+
+
+class LocalizedIsNullLookupsTestCase(TestCase):
+    """Tests whether ref lookups properly work with."""
+
+    TestModel1 = None
+
+    @classmethod
+    def setUpClass(cls):
+        """Creates the test model in the database."""
+        super(LocalizedIsNullLookupsTestCase, cls).setUpClass()
+        cls.TestModel = get_fake_model(
+            {"text": LocalizedField(null=True, required=[])}
+        )
+        cls.TestModel.objects.create(
+            text=LocalizedValue(dict(en="text_en", ro="text_ro", nl="text_nl"))
+        )
+        cls.TestModel.objects.create(
+            text=None,
+        )
+
+    def test_isnull_lookup_valid_values(self):
+        """Test whether isnull properly works with valid values."""
+        assert self.TestModel.objects.filter(text__isnull=True).exists()
+        assert self.TestModel.objects.filter(text__isnull=False).exists()
+
+    def test_isnull_lookup_null(self):
+        """Test whether isnull crashes with None as value."""
+
+        with pytest.raises(ValueError):
+            assert self.TestModel.objects.filter(text__isnull=None).exists()
+
+    def test_isnull_lookup_string(self):
+        """Test whether isnull properly works with string values on the
+        corresponding Django version."""
+        if django.VERSION < (4, 0):
+            assert self.TestModel.objects.filter(text__isnull="True").exists()
+        else:
+            with pytest.raises(ValueError):
+                assert self.TestModel.objects.filter(
+                    text__isnull="True"
+                ).exists()