python-utils-r1: Default to Python 3.6

Enabling Python 3.8 for SDK packages, but want to keep the default as
3.6 for now.

BUG=b:187794810
TEST=Pass CQ, successful SDK builder tryjob

Change-Id: I2343ae45917f8411e9b6201ffbb10da9a8a7d543
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/3895296
Tested-by: Sloan Johnson <sloanjohnson@google.com>
Commit-Queue: Sloan Johnson <sloanjohnson@google.com>
Auto-Submit: Sloan Johnson <sloanjohnson@google.com>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/overlays/eclass-overlay/+/41509
Reviewed-by: Oleksandr Tymoshenko <ovt@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>