(py-compute-indentation): When looking at a continuation line inside

(py-compute-indentation): When looking at a continuation line inside
an open paren, do a better job of reindenting the line.  For example:

def foo():
    print 'hello %s, %d' % (
    a, b)

Hit TAB on the line starting with `a'.  Without this patch this line
will never be reindented.