Fixed #28370 -- Deprecated the context arg of Field.from_db_value() and Expression.convert_value().

Unused since a0d166306f.

tests/from_db_value/test_deprecated.py

@@ -0,0 +1,22 @@
+import warnings
+
+from django.test import TestCase
+
+from .models import Cash, CashModelDeprecated
+
+
+class FromDBValueDeprecationTests(TestCase):
+
+    def test_deprecation(self):
+        CashModelDeprecated.objects.create(cash='12.50')
+        with warnings.catch_warnings(record=True) as warns:
+            warnings.simplefilter('always')
+            instance = CashModelDeprecated.objects.get()
+        self.assertIsInstance(instance.cash, Cash)
+        self.assertEqual(len(warns), 1)
+        msg = str(warns[0].message)
+        self.assertEqual(
+            msg,
+            'Remove the context parameter from CashFieldDeprecated.from_db_value(). '
+            'Support for it will be removed in Django 3.0.'
+        )