rewrite docs to explicitly use python2

I don't particularly care about Python 2 vs 3,
and don't think we're likely to leave 2 soon.

Bug: skia:9079
Change-Id: Ia28c1312309d4d9218d05915dcd21a45fe6f3727
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/213506
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Heather Miller <hcm@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
8 files changed