Use `is` to compare against sentinel object
Use `is` to compare against sentinel object

...since `==` can have repercussions by way of __eq__()

Change-Id: I668b11251a3d1192d42dfe914010d089df413da7

openstack/swift,swiftstack/swift,tipabu/swift,swiftstack/swift,tipabu/swift,tipabu/swift,swiftstack/swift,openstack/swift,openstack/swift,openstack/swift,tipabu/swift