stack/slapos: restore an old pin of importlib-metadata for py2
1c4a3c69 (stack/slapos.cfg: version up eggs (requests 2.31.0 and friends), 2023-10-04) broke component/nxdtest/test-py2.cfg and others using pytest on python2
1c4a3c69 (stack/slapos.cfg: version up eggs (requests 2.31.0 and friends), 2023-10-04) broke component/nxdtest/test-py2.cfg and others using pytest on python2