[Python-ideas] Improve error message when missing 'self' in method definition

[Stephen J. Turnbull]

Chris Angelico writes:

 > Given that it's not changing semantics at all, just adding info/hints
 > to an error message, it could well be added in a point release.

But it does change semantics, specifically for doctests.

I seem to recall that that is considered a blocker for this kind of
change in a maintenance-only branch.  In the end that's probably up to
the RM, but I would be mildly against it.  FWIW YMMV of course.