Always use python3 to run fetch-sk.

fetch-sk is no longer Python 2 compatible, and not all machines default
to Python3 (the Mac terminal still defaults to 2.7).

Change-Id: I6244ee591bbe787ef2dba46da2b20446265e42b8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/443406
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Ravi Mistry <rmistry@google.com>
Reviewed-by: Ravi Mistry <rmistry@google.com>
1 file changed