Change print statements to work in python3

This is part of a series of changes to introduce Python3 support.

Change-Id: I373be5de7141aa127d7debdbce1df39148dbec32

trace.py

@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from __future__ import print_function
 import sys
 import os
 REPO_TRACE = 'REPO_TRACE'
@@ -31,4 +32,4 @@ def SetTrace():
 
 def Trace(fmt, *args):
   if IsTrace():
-    print >>sys.stderr, fmt % args
+    print(fmt % args, file=sys.stderr)