Alter itertools import statement so that 2to3 fixes it.

dulwich/diff_tree.py

@@ -24,11 +24,7 @@ from collections import (
     )
 
 from io import BytesIO
-try:
-    from itertools import izip, chain
-except ImportError:
-    from itertools import chain
-    izip = zip
+from itertools import chain, izip
 import stat
 
 from dulwich.objects import (

dulwich/pack.py

@@ -39,11 +39,7 @@ from collections import (
     )
 import difflib
 
-try:
-    from itertools import chain, imap, izip
-except ImportError: # On Py3k the equivalents are the basic map and zip
-    from itertools import chain
-    imap, izip = map, zip
+from itertools import chain, imap, izip
 
 try:
     import mmap