On Windows, try py.exe if other Python executables fail.

[jcsteh]

1. Windows ships with python.exe and python3.exe to start the Windows Store app, even if the Store app isn't installed. We work around that by checking whether the Python executable actually works, not just checking whether it exists.
2. We still want to support users who explicitly put Python in the PATH, so we still try other Python executables first.
3. The actual python.exe isn't guaranteed to be in the PATH, but the py.exe launcher usually is, so we try that as a last resort. If we do use py.exe, we should end up with the user's preferred Python. Fixes #298.