installation: pinned PyTest
installation: pinned PyTest

Signed-off-by: Jiri Kuncar <2bdffcb416fed3002465740d19a053a39918cbe5@cern.ch>

inveniosoftware/invenio-i18n,inveniosoftware/invenio-i18n,inveniosoftware/invenio-i18n,inveniosoftware/invenio-i18n