Faster py-beg-of-def, ...-class, ...-top-level-form

New regexp `py-top-level-form-re': Matches a form which
starts at zero indent level, but is not a comment.
ert-tests files modified
`py--narrow-in-comint-modes' minor change
`py-down-base', go to the end of buffer if no match

`py-compute-indentation', slightly faster