Use mock.patch in migrations tests

Currently some of the migrations tests rely on the fact 'input' is aliased
because of six, instead of using mock.patch.  Replace this code with proper
use of mock.patch.

Also, replace one case of excessively specific python version check with
testing six.PY3

tests/auth_tests/test_tokens.py

@@ -1,4 +1,3 @@
-import sys
 import unittest
 from datetime import date, timedelta
 
@@ -6,6 +5,7 @@ from django.conf import settings
 from django.contrib.auth.models import User
 from django.contrib.auth.tokens import PasswordResetTokenGenerator
 from django.test import TestCase
+from django.utils.six import PY3
 
 
 class TokenGeneratorTest(TestCase):
@@ -54,7 +54,7 @@ class TokenGeneratorTest(TestCase):
         p2 = Mocked(date.today() + timedelta(settings.PASSWORD_RESET_TIMEOUT_DAYS + 1))
         self.assertFalse(p2.check_token(user, tk1))
 
-    @unittest.skipIf(sys.version_info[:2] >= (3, 0), "Unnecessary test with Python 3")
+    @unittest.skipIf(PY3, "Unnecessary test with Python 3")
     def test_date_length(self):
         """
         Make sure we don't allow overly long dates, causing a potential DoS.

tests/migrations/test_commands.py

@@ -8,7 +8,6 @@ import os
 from django.apps import apps
 from django.core.management import CommandError, call_command
 from django.db import DatabaseError, connection, models
-from django.db.migrations import questioner
 from django.test import ignore_warnings, mock, override_settings
 from django.utils import six
 from django.utils.deprecation import RemovedInDjango20Warning
@@ -535,8 +534,7 @@ class MakeMigrationsTests(MigrationTestBase):
         Makes sure that makemigrations enters and exits interactive mode properly.
         """
         # Monkeypatch interactive questioner to auto reject
-        old_input = questioner.input
-        questioner.input = lambda _: "N"
+        with mock.patch('django.db.migrations.questioner.input', mock.Mock(return_value='N')):
             try:
                 with self.temporary_migration_module(module="migrations.test_migrations_conflict") as migration_dir:
                     call_command("makemigrations", "migrations", merge=True, interactive=True, verbosity=0)
@@ -544,16 +542,13 @@ class MakeMigrationsTests(MigrationTestBase):
                     self.assertFalse(os.path.exists(merge_file))
             except CommandError:
                 self.fail("Makemigrations failed while running interactive questioner")
-        finally:
-            questioner.input = old_input
 
     def test_makemigrations_interactive_accept(self):
         """
         Makes sure that makemigrations enters interactive mode and merges properly.
         """
         # Monkeypatch interactive questioner to auto accept
-        old_input = questioner.input
-        questioner.input = lambda _: "y"
+        with mock.patch('django.db.migrations.questioner.input', mock.Mock(return_value='y')):
             out = six.StringIO()
             try:
                 with self.temporary_migration_module(module="migrations.test_migrations_conflict") as migration_dir:
@@ -562,8 +557,6 @@ class MakeMigrationsTests(MigrationTestBase):
                     self.assertTrue(os.path.exists(merge_file))
             except CommandError:
                 self.fail("Makemigrations failed while running interactive questioner")
-        finally:
-            questioner.input = old_input
             self.assertIn("Created new merge migration", force_text(out.getvalue()))
 
     def test_makemigrations_non_interactive_not_null_addition(self):
@@ -777,9 +770,8 @@ class MakeMigrationsTests(MigrationTestBase):
         behavior when --noinput is specified.
         """
         # Monkeypatch interactive questioner to auto reject
-        old_input = questioner.input
-        questioner.input = lambda _: "N"
         out = six.StringIO()
+        with mock.patch('django.db.migrations.questioner.input', mock.Mock(return_value='N')):
             try:
                 with self.temporary_migration_module(module="migrations.test_migrations_conflict") as migration_dir:
                     call_command("makemigrations", "migrations", merge=True, stdout=out)
@@ -788,8 +780,6 @@ class MakeMigrationsTests(MigrationTestBase):
                     self.assertFalse(os.path.exists(merge_file))
             except CommandError:
                 self.fail("Makemigrations failed while running interactive questioner")
-        finally:
-            questioner.input = old_input
             self.assertNotIn("Created new merge migration", out.getvalue())
 
     @override_settings(
@@ -817,8 +807,7 @@ class MakeMigrationsTests(MigrationTestBase):
         unspecified app even if it has conflicting migrations.
         """
         # Monkeypatch interactive questioner to auto accept
-        old_input = questioner.input
-        questioner.input = lambda _: "y"
+        with mock.patch('django.db.migrations.questioner.input', mock.Mock(return_value='y')):
             out = six.StringIO()
             try:
                 with self.temporary_migration_module(app_label="migrated_app") as migration_dir:
@@ -828,8 +817,6 @@ class MakeMigrationsTests(MigrationTestBase):
                 self.assertIn("No conflicts detected to merge.", out.getvalue())
             except CommandError:
                 self.fail("Makemigrations fails resolving conflicts in an unspecified app")
-        finally:
-            questioner.input = old_input
 
     def test_makemigrations_with_custom_name(self):
         """