Use /usr/bin/env python instead

R=dpranke@google.com

Bug: 1191100
Change-Id: I8de37ea7a02e7236d3bd36d4bae8cf4ffcf75ab9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2780631
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>