Let `textual run` run files that don't end in .py

[davep]

It's not uncommon to create "standalone" commands in Python that don't end in a .py and which:

#!/usr/bin/env python

or a variation on that theme. This commit makes it so that textual run will treat a file that's named and set to run like that as if it were a file that ends in .py.

See #1106.