my_activity.py talks to third party (e.g. review.coreboot.org). Without
certifi package, making https connection may result in
CERTIFICATE_VERIFY_FAILED.

Adding certifi package to .vpython (py2) doesn't work, so this change
drops py2 support for my_activity.py.

R=gavinmak@google.com

Fixed: 1255921
Change-Id: I515b14c81d37a52d482d08085d8029c791911ae8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3206906
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>