bpo-39816: More descriptive error msg than "too many values to unpack"

[ZackerySpytz]

https://bugs.python.org/issue39816

[ZackerySpytz]

If this approach is accepted, this PR would still need a news entry.

[aeros]

@csabella It looks like this PR is still in the draft stage, which typically is used by the author to indicate that it's not quite ready for review yet. Assuming @ZackerySpytz marks it as "Ready for Review" at some point, I'll try to look it over when I get the chance.

[rhettinger]

I don't think this should be done. It tightly couples the input side with the actual unpacking. Also, the message "too many value to unpack" is clear enough. I teach this to a new group of students every week and there is never a misunderstanding.

Marking this a closed. If someone wants to champion this, feel free to reopen. My opinion is that it isn't good design and that there isn't a real problem being solved.