Okay. But somehow I still think the current way of handling shebang is confusing. That makes a python3-only script unable to find a way to use both versioned shebang and virtualenv.

Maybe it could be changed to `Command is eligible for path search, or there is no version specification`.

Then if shebang is versioned, py.exe should try to search `PATH` for a versioned executable file(though it might not exist), and fallback to the installed list.

This could be less confusing I think.

And more importantly, virtualenv cannot and shouldn't create registry keys or even hijack py.exe to fix this issue. This change provides a better way.