Fixed #31842 -- Added DEFAULT_HASHING_ALGORITHM transitional setting.

It's a transitional setting helpful in migrating multiple instance of
the same project to Django 3.1+.

Thanks Markus Holtermann for the report and review, Florian
Apolloner for the implementation idea and review, and Carlton Gibson
for the review.

tests/deprecation/test_default_hashing_algorithm.py

@@ -0,0 +1,55 @@
+import sys
+from types import ModuleType
+
+from django.conf import (
+    DEFAULT_HASHING_ALGORITHM_DEPRECATED_MSG, Settings, settings,
+)
+from django.core.checks.security import base as security_base
+from django.test import TestCase, ignore_warnings
+from django.utils.deprecation import RemovedInDjango40Warning
+
+
+class DefaultHashingAlgorithmDeprecationTests(TestCase):
+    msg = DEFAULT_HASHING_ALGORITHM_DEPRECATED_MSG
+
+    def test_override_settings_warning(self):
+        with self.assertRaisesMessage(RemovedInDjango40Warning, self.msg):
+            with self.settings(DEFAULT_HASHING_ALGORITHM='sha1'):
+                pass
+
+    def test_settings_init_warning(self):
+        settings_module = ModuleType('fake_settings_module')
+        settings_module.SECRET_KEY = 'foo'
+        settings_module.DEFAULT_HASHING_ALGORITHM = 'sha1'
+        sys.modules['fake_settings_module'] = settings_module
+        try:
+            with self.assertRaisesMessage(RemovedInDjango40Warning, self.msg):
+                Settings('fake_settings_module')
+        finally:
+            del sys.modules['fake_settings_module']
+
+    def test_access(self):
+        # Warning is not raised on access.
+        self.assertEqual(settings.DEFAULT_HASHING_ALGORITHM, 'sha256')
+
+    @ignore_warnings(category=RemovedInDjango40Warning)
+    def test_system_check_invalid_value(self):
+        tests = [
+            None,
+            256,
+            'invalid',
+            'md5',
+            'sha512',
+        ]
+        for value in tests:
+            with self.subTest(value=value), self.settings(DEFAULT_HASHING_ALGORITHM=value):
+                self.assertEqual(
+                    security_base.check_default_hashing_algorithm(None),
+                    [security_base.E100],
+                )
+
+    @ignore_warnings(category=RemovedInDjango40Warning)
+    def test_system_check_valid_value(self):
+        for value in ['sha1', 'sha256']:
+            with self.subTest(value=value), self.settings(DEFAULT_HASHING_ALGORITHM=value):
+                self.assertEqual(security_base.check_default_hashing_algorithm(None), [])