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Description
At the moment the error is something like:
pluggy.manager.PluginValidationError: Plugin 'pytest_magic' could not be loaded: (mock 3.0.5 (/usr/local/lib/python2.7/dist-packages), Requirement.parse('mock<3,>=2.0.0; python_version < "3.3"'))!
When such conflicting requirement change happens we should enlist all packages that define requirements on that dependency, so the users can easier identify what package dependency conflicts with what another package dependency.