Add a new phase validating the usability of installed Python packages.
* gnu/packages/aux-files/python/sanity-check.py: New file.
* Makefile.am (AUX_FILES): Register it.
* guix/build-system/python.scm (sanity-check.py): New variable.
(lower): Add the script as an implicit input.
* guix/build/python-build-system.scm: Remove trailing #t.
(sanity-check): New phase.
(%standard-phases): Use it.
* tests/builders.scm: (make-python-dummy)
(dummy-ok, dummy-dummy-nosetuptools, dummy-fail-requirements)
(dummy-fail-import, dummy-fail-console-script): New variables.
("python-build-system: dummy-ok")
("python-build-system: dummy-dummy-nosetuptools")
("python-build-system: dummy-fail-requirements")
("python-build-system: dummy-fail-import")
("python-build-system: dummy-fail-console-script"): Add tests.
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