Update bloaty treemap to work on Python < 3.9.

`removeprefix` was only added to Python in March 2020. It isn't
available on Python 3.8.2, which is the default macOS Python installed
version.
https://www.python.org/dev/peps/pep-0616/

Change-Id: I7b8db0dfc55f1448cc72f41ba9a6ad5f099d48ef
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/448259
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
1 file changed