setup.py: Fix debian unstable package build

Apply fix as described in the package build log on unstable:

UserWarning: Distutils was imported before Setuptools, but importing
Setuptools also replaces the `distutils` module in `sys.modules`.
This may lead to undesirable behaviors or errors. To avoid these issues,
avoid using distutils directly, ensure that setuptools is installed in
the traditional way (e.g. not an editable install), and/or make sure that
setuptools is always imported before distutils.

See https://jenkins.softwareheritage.org/job/debian/job/packages/job/DSEA/job/gbp-buildpackage/89/console

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Migrated from D8490 (view on Phabricator)