[tox] Use `python -m check_manifest` instead of check-manifest program
To make sure we use Python from tox's virtualenv.
usr/lib/python2*/*-packages/cubicweb/devtools/
usr/lib/python2*/*-packages/cubicweb/skeleton/
usr/lib/python2*/*-packages/cubicweb/test
usr/lib/python2*/*-packages/cubicweb/dataimport/test
usr/lib/python2*/*-packages/cubicweb/entities/test
usr/lib/python2*/*-packages/cubicweb/ext/test
usr/lib/python2*/*-packages/cubicweb/server/test
usr/lib/python2*/*-packages/cubicweb/sobjects/test
usr/lib/python2*/*-packages/cubicweb/hooks/test
usr/lib/python2*/*-packages/cubicweb/web/test
usr/lib/python2*/*-packages/cubicweb/etwist/test