Add support for Debian bullseye which runs python3.9.
This was tested by running tox locally on a Debian bullseye
VM.
Story: 2006796
Task: 42931,42925,43099
Signed-off-by: Charles Short <charles.short@windriver.com>
Change-Id: I2b18437db2249ecd7b358c6d1e49023afa3da21a