Use hashlib module instead of md5 if it is present.

tools/presubmit.py

Index: tools/presubmit.py
diff --git a/tools/presubmit.py b/tools/presubmit.py
index 04952e0b4a232e3e9cc1f3c857cccd96d576b3ef..e69c9a85ac8e3ee033779c2f2847b3061abbf226 100755
--- a/tools/presubmit.py
+++ b/tools/presubmit.py
@@ -27,8 +27,14 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+try:
+  import hashlib
+  md5er = hashlib.md5
+except ImportError, e:
+  import md5
+  md5er = md5.new
+
 
-import md5
 import optparse
 import os
 from os.path import abspath, join, dirname, basename, exists
@@ -126,7 +132,7 @@ class FileContentsCache(object):
     for file in files:
       try:
         handle = open(file, "r")
-        file_sum = md5.new(handle.read()).digest()
+        file_sum = md5er(handle.read()).digest()
         if not file in self.sums or self.sums[file] != file_sum:
           changed_or_new.append(file)
           self.sums[file] = file_sum