Added python_version() function + struct

solves issue #6 
This feels a bit overly complex for its purpose. As far as I know, all version instances in a program will be identical by definition, so we don't need to provide a constructor with parameters. The no-parameter constructor should just set the members to the correct values and be done.

Along with that, the members themselves should be const. Unless I'm missing some use case, this class should be immutable and single-purposed.