You should consider upgrading via the 'pip install --upgrade pip' command.
$ git clone https://gitlab.com/BuildStream/buildstream.git
Cloning into 'buildstream'...
fatal: unable to access 'https://gitlab.com/BuildStream/buildstream.git/': The requested URL returned error: 503
section_end:1533988056:build_script
section_start:1533988056:after_script
section_end:1533988058:after_script
section_start:1533988058:upload_artifacts
Uploading artifacts...
WARNING: logs: no matching files
WARNING: project.refs: no matching files
Uploading artifacts to coordinator... ok id=82225 responseStatus=201 Created token=TrzzCuyz
section_end:1533988062:upload_artifacts
ERROR: Job failed: exit code 1