[3.12] gh-104271: Fix auto() fallback in case of mixed type Enum

[itamaro]

gh-91457 changed the behavior of auto() in case of mixed type Enums.
the fallback behavior (before deprecation) was supposed to be backwards compatible (I think), but iterating the sorted last_values in order (instead of reversed order) results an incorrect behavior of always reusing values after the first auto(), as demonstrated in the test case I extended in this PR.

• Issue: Enum works in python 3.10 breaks in 3.11 #104271

[miss-islington]

Thanks @itamaro for the PR, and @ethanfurman for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11.
🐍🍒⛏🤖

[miss-islington]

Sorry @itamaro and @ethanfurman, I had trouble checking out the 3.11 backport branch.
Please retry by removing and re-adding the "needs backport to 3.11" label.
Alternatively, you can backport using cherry_picker on the command line.
cherry_picker f4e2049f14d40c1b893c68530eec5e341cf3d929 3.11

[itamaro]

@ethanfurman how do we port it also to main branch now? should it be a new PR for main?

[ethanfurman]

It doesn't need to go in main (which is now 3.13) because mixed types are no longer supported by the default _generate_next_value_().

[miss-islington]

Thanks @itamaro for the PR, and @ethanfurman for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11.
🐍🍒⛏🤖

[bedevere-bot]

GH-104809 is a backport of this pull request to the 3.11 branch.