> "Python Launcher is sorry to say... No pyvenv.cfg file" 

As already mentioned, that it fails with this error means you're running a virtual-environment launcher instead of a base Python executable. That it displays a message box means you ran a GUI pythonw.exe virtual-environment launcher. pyvenv.cfg is a configuration file that the virtual-environment launcher looks for beside the executable, or one directory up. It tells the launcher where to find the base python[w].exe executable.

As already suggested, you should associate .py[w] scripts with the "Python" app that depicts a rocket/shuttle being launched on its icon. This will associate .py[w] scripts with the py[w].exe launcher, and it also associates them with a shell drop handler that allows files to be dragged and dropped on a script icon in Explorer.