Fix use of 'is' operator for comparison

The 'is' operator is not meant to be used for comparisons. It currently working is an implementation detail of CPython.
CPython 3.8 has added a SyntaxWarning for this.

Bug: chromium:958874
Change-Id: I7657bd3fddf37e849cc30aca34825fa3c0f71b58
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/211663
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Raul Tambre <raul@tambre.ee>
3 files changed