Improving the code quality of upstream by adding the flake8 check on
zuul and adding the flake8-import-order plugin to standardize imports.
Also, defaults testenv to python3.9 configuration that should be used
for now on with the debian migration.
Story: 2010100
Task: 45669
Signed-off-by: Thiago Brito <thiago.brito@windriver.com>
Change-Id: I55aa952c4f22a7af53e1f1c11a4a51997afa4bcf