Tests are failing against bst-plugins-experimental master

Job #422860981 failed for 47fb4297:

The relevant bit from the error message:

INTERNALERROR> pkg_resources.VersionConflict: (BuildStream 1.93.0+18.g47fb4297 (/builds/BuildStream/buildstream/src), Requirement.parse('buildstream>=1.93.0.dev0<2.0.0'))