[entropy.client.interfaces.dep] restore python3 compatibility
[entropy.spm] PortagePlugin: make sure that /usr/lib/portage/pym is in sys.path
[entropy.*] create new dependencies metadata "pkg_dependencies"

The old "dependencies" metadata is deprecated. It was found that
the generated metadata might get corrupted by colliding atom strings.
The new implementation avoids collisions completely and is more
efficient.
[entropy.spm] PortagePlugin.__source_env_get_var: silence stderr