spack-python should exec `spack python` (#15738)

The current implementation of `spack-python` will leave an extra shell
around while it runs.  That shell should really replace itself with
spack.

- [x] add exec to spack-python script

bin/spack-python

@@ -22,4 +22,4 @@
 #
 # This is compatible across platforms.
 #
-/usr/bin/env spack python "$@"
+exec /usr/bin/env spack python "$@"