stack/erp5,software/erp5: support python3

This brings updates of many dependencies on python3, while mostly
keeping the same versions for python2.

This enables a new `erp5-py3` test in software/slapos-sr-testing, which
should pass already, but with NEO and WCFS being marked as expected
failure or skipped for now.

This also brings pylint support on python3, which we will be able to
reuse in software/slapos-testing.

Co-authored-by: Kazuhiko SHIOZAKI <kazuhiko@nexedi.com>
Co-authored-by: Arnaud Fontaine <arnaud.fontaine@nexedi.com>
Co-authored-by: Bryton Lacquement <bryton.lacquement@nexedi.com>