In PEP517 mode setup.py is sourced - not executed - and the build fails
with ImportError like this:

    Preparing wheel metadata ... error
    ERROR: Command errored out with exit status 1:
     command: /home/kirr/src/wendelin/venv/z-dev/bin/python2 /home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py prepare_metadata_for_build_wheel /tmp/tmp2F3aEs
         cwd: /home/kirr/src/wendelin/wendelin.core
    Complete output (53 lines):
    running dist_info
    creating /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info
    writing requirements to /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/requires.txt
    writing /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/PKG-INFO
    writing top-level names to /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/top_level.txt
    writing dependency_links to /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/dependency_links.txt
    writing entry points to /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/entry_points.txt
    writing manifest file '/tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/SOURCES.txt'
    package init file '__init__.py' not found (or not a regular file)
    Traceback (most recent call last):
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 257, in <module>
        main()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 240, in main
        json_out['return_val'] = hook(**hook_input['kwargs'])
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 110, in prepare_metadata_for_build_wheel
        return hook(metadata_directory, config_settings)
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/build_meta.py", line 155, in prepare_metadata_for_build_wheel
        self.run_setup()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/build_meta.py", line 234, in run_setup
        self).run_setup(setup_script=setup_script)
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/build_meta.py", line 141, in run_setup
        exec(compile(code, __file__, 'exec'), locals())
      File "setup.py", line 374, in <module>
        """.splitlines()]
      File "/home/kirr/src/tools/go/pygolang/golang/pyx/build.py", line 118, in setup
        setuptools_dso.setup(**kw)
      File "/home/kirr/src/tools/py/pypa/setuptools_dso/src/setuptools_dso/__init__.py", line 37, in setup
        _setup(**kws)
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/__init__.py", line 145, in setup
        return distutils.core.setup(**attrs)
      File "/usr/lib/python2.7/distutils/core.py", line 151, in setup
        dist.run_commands()
      File "/usr/lib/python2.7/distutils/dist.py", line 953, in run_commands
        self.run_command(cmd)
      File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
        cmd_obj.run()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/dist_info.py", line 31, in run
        egg_info.run()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 296, in run
        self.find_sources()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 303, in find_sources
        mm.run()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 534, in run
        self.add_defaults()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 574, in add_defaults
        rcfiles = list(walk_revctrl())
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/sdist.py", line 20, in walk_revctrl
        for item in ep.load()(dirname):
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2434, in load
        return self.resolve()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2444, in resolve
        raise ImportError(str(exc))
    ImportError: 'module' object has no attribute 'git_lsfiles'

See comments added to register_as_entrypoint for explanation of what
happens.

Wendelin.core will soon switch to PEP517 mode (by adding pyproject.toml)
to build-require Cython, Pygolang and friends.