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We were generating a test profile using `extra-paths` for manuel and
six, that are pre-installed in software, but this did not prevent
buildout from the test instance from installing latest version of manuel
on python2. Since that version is incompatible with py2, tests on py2
were no longer running.

Switch to a simpler approach of generating a profile with the versions
that we already have, assuming that if we could install them once we
should be able to install the same versions again.