1. I don't think searching . should be included - on Unix /usr/bin/env searches PATH, and I believe we should do the same here.

2. The PATH search does happen (from my reading of the code) but it looks for a "python3" command, which isn't available. Again this is the same behaviour as Unix, and so defensible, but given that Windows doesn't provide the versioned executables, it's less useful there.

The biggest problem is that with "#!/usr/bin/env python3" the user clearly expects Python 3, and without versioned executables, we can't guarantee that on Windows for a PATH search. Whether not supporting this usage is worse than supporting it without a guarantee that you'll get Python 3, I'm not sure.