environment: Use python3 instead of python

Change-Id: I9d70b8d4d71bdebb4ce91e04b6f5cc8c61073360
Bug: 358
Reviewed-on: https://pigweed-review.googlesource.com/c/infra/recipes/+/41260
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Oliver Newman <olivernewman@google.com>
5 files changed