Fixed that Python requirement gets into package metadata

[andy-maier]

This addresses issue #464.

Details:

• The setup.cfg file already had a "python_requires" attribute, but it was in the "options.entry_points" section, from where it did not make its way into the package metadata. This fix moves the "python_requires" attribute into the "options" section. which should cause the package build process to add it to the package metadata.