Docker build failing on latest master

[HofmT]

I'm seeing this error when trying to run docker build . on the latest master (latest commit c753c78)

#13 10.95 Collecting shyaml
#13 11.39   Downloading https://files.pythonhosted.org/packages/2a/5c/139cda90758d6bc18a16876a2c291264cf06fd736ca54606862a43931877/shyaml-0.6.2-py2.py3-none-any.whl
#13 11.42 Collecting pyyaml (from shyaml)
#13 11.59   Downloading https://files.pythonhosted.org/packages/a0/a4/d63f2d7597e1a4b55aa3b4d6c5b029991d3b824b5bd331af8d4ab1ed687d/PyYAML-5.4.1.tar.gz (175kB)
#13 11.81   Installing build dependencies: started
#13 14.99   Installing build dependencies: finished with status 'done'
#13 15.14     Complete output from command python setup.py egg_info:
#13 15.14     running egg_info
#13 15.14     creating pip-egg-info/PyYAML.egg-info
#13 15.14     writing pip-egg-info/PyYAML.egg-info/PKG-INFO
#13 15.14     writing top-level names to pip-egg-info/PyYAML.egg-info/top_level.txt
#13 15.14     writing dependency_links to pip-egg-info/PyYAML.egg-info/dependency_links.txt
#13 15.14     writing manifest file 'pip-egg-info/PyYAML.egg-info/SOURCES.txt'
#13 15.14     Traceback (most recent call last):
#13 15.14       File "<string>", line 1, in <module>
#13 15.14       File "/tmp/pip-install-ptcLCZ/pyyaml/setup.py", line 295, in <module>
#13 15.14         python_requires='>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, !=3.5.*',
#13 15.14       File "/tmp/pip-build-env-ziD7W8/lib/python2.7/site-packages/setuptools/__init__.py", line 162, in setup
#13 15.14         return distutils.core.setup(**attrs)
#13 15.14       File "/usr/lib/python2.7/distutils/core.py", line 151, in setup
#13 15.14         dist.run_commands()
#13 15.14       File "/usr/lib/python2.7/distutils/dist.py", line 953, in run_commands
#13 15.14         self.run_command(cmd)
#13 15.14       File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
#13 15.14         cmd_obj.run()
#13 15.14       File "/tmp/pip-build-env-ziD7W8/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 296, in run
#13 15.14         self.find_sources()
#13 15.14       File "/tmp/pip-build-env-ziD7W8/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 303, in find_sources
#13 15.14         mm.run()
#13 15.14       File "/tmp/pip-build-env-ziD7W8/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 534, in run
#13 15.14         self.add_defaults()
#13 15.14       File "/tmp/pip-build-env-ziD7W8/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 570, in add_defaults
#13 15.14         sdist.add_defaults(self)
#13 15.14       File "/tmp/pip-build-env-ziD7W8/lib/python2.7/site-packages/setuptools/command/py36compat.py", line 36, in add_defaults
#13 15.14         self._add_defaults_ext()
#13 15.14       File "/tmp/pip-build-env-ziD7W8/lib/python2.7/site-packages/setuptools/command/py36compat.py", line 120, in _add_defaults_ext
#13 15.14         self.filelist.extend(build_ext.get_source_files())
#13 15.14       File "/tmp/pip-install-ptcLCZ/pyyaml/setup.py", line 201, in get_source_files
#13 15.14         self.cython_sources(ext.sources, ext)
#13 15.14       File "/usr/lib/python2.7/distutils/cmd.py", line 105, in __getattr__
#13 15.14         raise AttributeError, attr
#13 15.14     AttributeError: cython_sources
#13 15.14
#13 15.14     ----------------------------------------
#13 15.16 Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-install-ptcLCZ/pyyaml/

Might be related to yaml/pyyaml#601 ?