Fix the import of Tkinter in Python 3+

scripts/python/openbabel/pybel.py

@@ -51,7 +51,7 @@ else:
     from . import openbabel as ob
     _obfuncs = _obconsts = ob
     try:
-        import Tkinter as tk
+        from six.moves import tkinter as tk
         from PIL import Image as PIL
         from PIL import ImageTk as piltk
     except ImportError:  # pragma: no cover

scripts/python/setup.py

@@ -126,6 +126,7 @@ setup(
     zip_safe=False,
     cmdclass={'build': CustomBuild, 'build_ext': CustomBuildExt, 'install': CustomInstall, 'sdist': CustomSdist},
     packages=['openbabel'],
+    install_requires=[six],
     ext_modules=[obextension],
     classifiers=[
         'Development Status :: 5 - Production/Stable',