title: Python Error when installing ruamel.yaml package with pip
body: <p>I am receiving an error when I want to install the ruamel.yaml package</p>
  
  <p>Here is the command: </p>
  
  <pre><code>python --version;pip --version;pip install ruamel.yaml
  </code></pre>
  
  <p>And the output: </p>
  
  <pre><code>Python 2.7.10
  pip 10.0.0 from /Library/Python/2.7/site-packages/pip (python 2.7)
  Collecting ruamel.yaml
    Downloading https://files.pythonhosted.org/packages/c1/ad/9557a78f1b61951623c56225e05dacdd81f80519f13fc39a926a5793edfa/ruamel.yaml-0.15.37.tar.gz (280kB)
      100% |████████████████████████████████| 286kB 4.4MB/s
      Complete output from command python setup.py egg_info:
      sys.argv ['-c', 'egg_info', '--egg-base', 'pip-egg-info']
      Traceback (most recent call last):
        File "&lt;string&gt;", line 1, in &lt;module&gt;
        File "/private/var/folders/8j/69k5q3yn4y51g5nt2g7g9b9n24s68p/T/pip-install-htyM8C/ruamel.yaml/setup.py", line 917, in &lt;module&gt;
          main()
        File "/private/var/folders/8j/69k5q3yn4y51g5nt2g7g9b9n24s68p/T/pip-install-htyM8C/ruamel.yaml/setup.py", line 905, in main
          setup(**kw)
        File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/distutils/core.py", line 111, in setup
          _setup_distribution = dist = klass(attrs)
        File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/setuptools/dist.py", line 272, in __init__
          _Distribution.__init__(self,attrs)
        File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/distutils/dist.py", line 287, in __init__
          self.finalize_options()
        File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/setuptools/dist.py", line 326, in finalize_options
          ep.require(installer=self.fetch_build_egg)
        File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources/__init__.py", line 2385, in require
          reqs = self.dist.requires(self.extras)
        File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources/__init__.py", line 2617, in requires
          dm = self._dep_map
        File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources/__init__.py", line 2606, in _dep_map
          if invalid_marker(marker):
        File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources/__init__.py", line 1424, in is_invalid_marker
          cls.evaluate_marker(text)
        File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources/__init__.py", line 1549, in _markerlib_evaluate
          env = cls._translate_metadata2(_markerlib.default_environment())
        File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources/__init__.py", line 1537, in _translate_metadata2
          for key, value in env
        File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources/__init__.py", line 1536, in &lt;genexpr&gt;
          (key.replace('.', '_'), value)
      ValueError: too many values to unpack
  
      ----------------------------------------
  Command "python setup.py egg_info" failed with error code 1 in /private/var/folders/8j/69k5q3yn4y51g5nt2g7g9b9n24s68p/T/pip-install-htyM8C/ruamel.yaml/
  </code></pre>
  
  <p>To give a bit of a background. I installed python3 with brew and I uninstalled it thereafter. </p>
  
  <p>Now I cannot install ruamel.yaml </p>
  
  <p>Has anybody had the same issue?
  How did you solve it?</p>