Fix the conflict status with hacking.

The conflict is caused by:
    The user requested flake8 3.8.3 (from /home/zuul/.cache/pre-commit/repozi7hytk7)
    hacking 3.0.1 depends on flake8<3.8.0 and >=3.6.0

Install the latest version of hacking to fix the conflict error with the
pre-commit hook.

Change-Id: If1c54502e6e10751fd722694aa5429a08f376798

.pre-commit-config.yaml

@@ -32,4 +32,4 @@ repos:
     hooks:
       - id: flake8
         additional_dependencies:
-          - hacking>=3.0.1,<3.1.0
+          - hacking==4.0.0

oslo_db/tests/sqlalchemy/test_exc_filters.py

@@ -663,19 +663,11 @@ class TestExceptionCauseMySQLSavepoint(test_base._MySQLOpportunisticTestCase):
                     # session even though the SAVEPOINT was lost;
                     # the net result here is that one exception is thrown
                     # instead of two.  This is SQLAlchemy ticket #3680
-                    self.assertTrue(
-                        isinstance(
-                            dbe_inner.cause,
-                            exception.DBDuplicateEntry
-                        )
-                    )
+                    self.assertIsInstance(
+                        dbe_inner.cause, exception.DBDuplicateEntry)
+
         except exception.DBError as dbe_outer:
-            self.assertTrue(
-                isinstance(
-                    dbe_outer.cause,
-                    exception.DBDuplicateEntry
-                )
-            )
+            self.AssertIsInstance(dbe_outer.cause, exception.DBDuplicateEntry)
 
         # resets itself afterwards
         try:

oslo_db/tests/sqlalchemy/test_fixtures.py

@@ -250,9 +250,7 @@ class TestLoadHook(unittest.TestCase):
         found_tests = loader.discover(start_dir, pattern="test_fixtures.py")
         new_loader = load_tests(loader, found_tests, "test_fixtures.py")
 
-        self.assertTrue(
-            isinstance(new_loader, testresources.OptimisingTestSuite)
-        )
+        self.assertIsInstance(new_loader, testresources.OptimisingTestSuite)
 
         actual_tests = unittest.TestSuite(
             testscenarios.generate_scenarios(found_tests)
@@ -274,9 +272,7 @@ class TestLoadHook(unittest.TestCase):
         new_loader = load_tests(
             loader, unittest.suite.TestSuite(), "test_fixtures.py")
 
-        self.assertTrue(
-            isinstance(new_loader, testresources.OptimisingTestSuite)
-        )
+        self.assertIsInstance(new_loader, testresources.OptimisingTestSuite)
 
         actual_tests = unittest.TestSuite(
             testscenarios.generate_scenarios(