Gpyutils

- support listing maintainers in gpy-upgrade-impl

- new `gpy-py2` tool to aid finding py2-only packages

- eclasses: fix recognizing single-impl distutils-r1
- gpy-showimpls: remove trailing padding

- gpy-drop-dead-impls: fix error handling
- implementations: support new python-single-r1 logic

- gpy-drop-dead-impls: remove broken PYTHON_COMPAT guessing optimization
- remove gpy-find-missing-meta (the reports are now in pkgcheck)
- gpy-depgraph: support getting a list of deps for a package
- gpy-upgrade-impl: support getting deps needing upgrading for a package
- support BDEPEND (requires gentoopm 0.3.1)

- gpy-impl: fixed usage errors
- mode is now preserved while mangling PYTHON_COMPAT
- gpy-depgraph: refactored code to ease adjusting it to custom purposes