In 12.0 it was 3.5 but for 13.0, it is acceptable 3.6.
Debian Buster has 3.7 and Ubuntu 18.04 LTS has 3.6
This will avoid undeterminist errors with OrderedDict

CentOS 7 still does not ship python3 by default but it is possible to
install it and CentOS 8 with python 3.6 was released in septembre 2019

closes odoo/odoo#38827

X-original-commit: 14e3e752
Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>