[DRE-maint] gitlab-shell is marked for autoremoval from testing

[Debian testing autoremoval watch]

gitlab-shell 12.0.0-2 is marked for autoremoval from testing on 2020-05-15

It (build-)depends on packages with these RC bugs:
955064: grpc: FTBFS with Sphinx 2.4: distutils.errors.DistutilsError: Could not find suitable distribution for Requirement.parse('Sphinx~=1.8.1')