upload: Fix tests for object identity to use 'is not'

flake8 reports:

  E714 test for object identity should be 'is not'

Change-Id: Ib8c4100babaf952bbfe65fd56555ece8a958e4b0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254450
Tested-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>