Bug reported by Antoine Pitrou on the python-committers mailing list.

According to Brett Canon, the first failure was:
https://travis-ci.org/python/cpython/jobs/228409786

And the regression is related to my commit a5c62a8e9f0de6c4133825a5710984a3cd5e102b: bpo-23404.

Collecting coverage
  Downloading coverage-4.3.4.tar.gz (361kB)
�    Complete output from command python setup.py egg_info:
    running egg_info
    creating pip-egg-info/coverage.egg-info
    writing pip-egg-info/coverage.egg-info/PKG-INFO
    writing dependency_links to
pip-egg-info/coverage.egg-info/dependency_links.txt
    writing entry points to pip-egg-info/coverage.egg-info/entry_points.txt
    writing top-level names to pip-egg-info/coverage.egg-info/top_level.txt
    writing manifest file 'pip-egg-info/coverage.egg-info/SOURCES.txt'
    warning: manifest_maker: standard file '-c' not found

    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-build-ewtgqc2r/coverage/setup.py", line 204, in
<module>
        main()
      File "/tmp/pip-build-ewtgqc2r/coverage/setup.py", line 194, in main
        setup(**setup_args)
      File "/home/travis/build/python/cpython/Lib/distutils/core.py",
line 148, in setup
        dist.run_commands()
      File "/home/travis/build/python/cpython/Lib/distutils/dist.py",
line 955, in run_commands
        self.run_command(cmd)
      File "/home/travis/build/python/cpython/Lib/distutils/dist.py",
line 974, in run_command
        cmd_obj.run()
      File
"/home/travis/build/python/cpython/venv/lib/python3.7/site-packages/setuptools/command/egg_info.py",
line 279, in run
        self.find_sources()
      File
"/home/travis/build/python/cpython/venv/lib/python3.7/site-packages/setuptools/command/egg_info.py",
line 306, in find_sources
        mm.run()
      File
"/home/travis/build/python/cpython/venv/lib/python3.7/site-packages/setuptools/command/egg_info.py",
line 533, in run
        self.add_defaults()
      File
"/home/travis/build/python/cpython/venv/lib/python3.7/site-packages/setuptools/command/egg_info.py",
line 562, in add_defaults
        sdist.add_defaults(self)
      File
"/home/travis/build/python/cpython/Lib/distutils/command/sdist.py", line
228, in add_defaults
        self._add_defaults_ext()
      File
"/home/travis/build/python/cpython/Lib/distutils/command/sdist.py", line
311, in _add_defaults_ext
        build_ext = self.get_finalized_command('build_ext')
      File "/home/travis/build/python/cpython/Lib/distutils/cmd.py",
line 299, in get_finalized_command
        cmd_obj.ensure_finalized()
      File "/home/travis/build/python/cpython/Lib/distutils/cmd.py",
line 107, in ensure_finalized
        self.finalize_options()
      File
"/home/travis/build/python/cpython/Lib/distutils/command/build_ext.py",
line 150, in finalize_options
        py_include = sysconfig.get_python_inc()
      File
"/home/travis/build/python/cpython/Lib/distutils/sysconfig.py", line
100, in get_python_inc
        incdir = os.path.join(_sys_home, get_config_var('AST_H_DIR'))
      File "/home/travis/build/python/cpython/Lib/posixpath.py", line
92, in join
        genericpath._check_arg_types('join', a, *p)
      File "/home/travis/build/python/cpython/Lib/genericpath.py", line
149, in _check_arg_types
        (funcname, s.__class__.__name__)) from None
    TypeError: join() argument must be str or bytes, not 'NoneType'

    ----------------------------------------
�Command "python setup.py egg_info" failed with error code 1 in
/tmp/pip-build-ewtgqc2r/coverage/�