Adjust Postgres indexes for Django 3.2 (#616)

tests/typecheck/contrib/postgres/test_indexes.yml

@@ -0,0 +1,18 @@
+-   case: can_instantiate_index
+    main: |
+        from django.db import models
+        from django.db.models.functions import Lower
+        from django.contrib.postgres.indexes import {{ index }}
+        {{ index }}(fields=["foo"])
+        {{ index }}(models.F("bar"), name="test")
+        {{ index }}("foo", models.F("bar"), Lower("baz"), name="test")
+        {{ index }}("foo", name="test", opclasses=["bar"])
+        {{ index }}("foo", name="test", condition=models.Q(foo=1))
+        {{ index }}("foo", name="test", include=["bar"])
+    parametrized:
+    -   index: BrinIndex
+    -   index: BTreeIndex
+    -   index: GinIndex
+    -   index: GistIndex
+    -   index: HashIndex
+    -   index: SpGistIndex