Update docs to reference python3

Also updates a script with 2to3 to be python3-compatible.

Change-Id: I45981349664aef4491e7ab20e49889d461f8bf3f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/538417
Commit-Queue: James Godfrey-Kittle <jamesgk@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
7 files changed