docker-py does not install

Installation of the docker-py package, via pip, has suddenly stopped working. The error:

fatal: [localhost]: FAILED! => {"changed": false, "cmd": "/usr/bin/pip2 install docker-py==1.10.6", "failed": true, "msg": "stdout: Downloading/unpacking docker-py==1.10.6\nDownloading/unpacking doc
ker-pycreds>=0.2.1 (from docker-py==1.10.6)\n Downloading docker_pycreds-0.2.1-py2.py3-none-any.whl\nRequirement already satisfied (use --upgrade to upgrade): six>=1.4.0 in /usr/lib/python2.7/dist-packages (fro
m docker-py==1.10.6)\nDownloading/unpacking requests>=2.5.2,!=2.11.0 (from docker-py==1.10.6)\nCleaning up...\nException:\nTraceback (most recent call last):\n File \"/usr/lib/python2.7/dist-packages/pip/baseco
mmand.py\", line 122, in main\n status = self.run(options, args)\n File \"/usr/lib/python2.7/dist-packages/pip/commands/install.py\", line 278, in run\n requirement_set.prepare_files(finder, force_root_eg
g_info=self.bundle, bundle=self.bundle)\n File \"/usr/lib/python2.7/dist-packages/pip/req.py\", line 1266, in prepare_files\n req_to_install.extras):\n File \"/usr/lib/python2.7/dist-packages/pkg_resources.
py\", line 2291, in requires\n dm = self.dep_map\n File \"/usr/lib/python2.7/dist-packages/pkg_resources.py\", line 2484, in _dep_map\n self._dep_map = self._compute_dependencies()\n File \"/usr/lib/py
thon2.7/dist-packages/pkg_resources.py\", line 2517, in _compute_dependencies\n common = frozenset(reqs_for_extra(None))\n File \"/usr/lib/python2.7/dist-packages/pkg_resources.py\", line 2514, in reqs_for_e
xtra\n if req.marker_fn(override=

):\n File \"/usr/share/python-wheels/setuptools-3.3-py2.py3-none-any.whl/_markerlib/markers.py\", line 113, in marker_fn\n return eval(compiled_marker, envi
ronment)\n File \"<environment marker>\", line 1, in <module>\nNameError: name 'platform_system' is not defined\n\nStoring debug log for failure in /root/.pip/pip.log\n"}