Make sure base class setup method is called for a few tests.

dulwich/tests/test_pack.py

@@ -733,6 +733,7 @@ class TestPackIterator(DeltaChainIterator):
 class DeltaChainIteratorTests(TestCase):
 
     def setUp(self):
+        super(DeltaChainIteratorTests, self).setUp()
         self.store = MemoryObjectStore()
         self.fetched = set()
 

dulwich/tests/test_utils.py

@@ -37,6 +37,7 @@ from utils import (
 class BuildCommitGraphTest(TestCase):
 
     def setUp(self):
+        super(BuildCommitGraphTest, self).setUp()
         self.store = MemoryObjectStore()
 
     def test_linear(self):

dulwich/tests/test_walk.py

@@ -74,6 +74,7 @@ class TestWalkEntry(object):
 class WalkerTest(TestCase):
 
     def setUp(self):
+        super(WalkerTest, self).setUp()
         self.store = MemoryObjectStore()
 
     def make_commits(self, commit_spec, **kwargs):