Default to `python` when `uv run` does not recieve a command (#3109)
This means that a bare `uv run` invocation drops you into a REPL.
This behavior is internally controversial, and may best be served by a
dedicated `uv repl` command. I would imagine it's important to fail if
no command is given in _some_ circumstances, but those may be resolved
by _not_ doing this if we do not detect a TTY.
Regardless, I'm interested in giving this a try for a bit during this
experimental phase.
