support/misc/gitlab-ci.yml.in: use python3 for flake8

The pkg-stats script now uses Python3 only constructs (the "async"
keyword) and therefore fails to pass the Python2 flake8 test.

Let's use the Python3 flake8 instead.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/681711009



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Titouan Christophe <titouan.christophe@railnova.eu>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>