Make using the global Python environment opt-in

[brettcannon]

This would probably entail creating an empty virtual environment on execution to avoid using anything installed globally (this is effectively #329 but with an assumption every file defines at least an empty set of dependencies and a Python requirement of anything).There should be some form of opt-out to allow using the global environment for some reason.

This would help prevent installing globally or accidentally relying on something installed globally.

[brettcannon]

Related: #243

[brettcannon]

This probably should be a part of a py run command which makes the interpreter selection an implementation detail.