Refine check for `self` in magic_check_fn.
Refine check for `self` in magic_check_fn.

In a future CL there is a protobuf object on the stack whose __eq__
method freaks out during this comparison.

Regardless, the 'is' comparison will always be faster and safer too.

R=gbeaty@chromium.org, a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org

Change-Id: I3aacea5fcacbf55d2166a622614c2c725e65872b
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/1626477
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Auto-Submit: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>

luci/recipes-py,luci/recipes-py