I am proposing this change here, because I am aware it might be
controversial.  My motivation is to have more consistent inferior modes,
and not have to be surprised that (as in my case right now,
inferior-python-mode doesn't allow be to kill a prompt).  I haven't
written a NEWS entry yet, but it should probably mention that anyone
interested in restoring the old behaviour should just set
'comint-prompt-read-only' directly.

WDYT?