stack/slapos: add distro version pin for upcoming slapos.core release

In slapos.core commit 425f5bb7b (slapgrid: replace platform.linux_distribution()
with distro.linux_distribution(), 2022-03-21), a dependency to distro
was introduced. Because tests run with slapos.core fetched from git, they
already started to fail because of the missing version pin.

The pin is made to the latest version for both python2 and python3.
Usually we take the latest version compatible with python2, but I just
thought this way was probably better.