Fix some python2-isms in prep for python3

These ones should work in both versions so we don't have to do them as
part of a global change over.

Change-Id: I2177b329d385c50d706a50a750b6c115ea3ea7ae
diff --git a/aos/vision/download/downloader.py b/aos/vision/download/downloader.py
index 3aea634..be8fda2 100644
--- a/aos/vision/download/downloader.py
+++ b/aos/vision/download/downloader.py
@@ -34,7 +34,7 @@
   jetson_fnames = [ToJetsonFname(fname) for fname in fnames]
   checksums = GetChecksums(fnames)
   jetson_checksums = GetJetsonChecksums(ssh_target, jetson_fnames)
-  for i in xrange(len(fnames)):
+  for i in range(len(fnames)):
     if (checksums[i] != jetson_checksums[i]):
       # if empty, unlink
       subprocess.check_call(["ssh", ssh_target, "unlink " + jetson_fnames[i]])