Bug report

The following script changes output between 3.10.4 (and earlier) to 3.11.0b1.

import enum
class E(str, enum.Enum):
    A = "A"
print("hello {0}".format(E.A))

$ python3.10 test.py
hello A
$ python3.11 test.py
hello E.A

Per https://docs.python.org/3/library/enum.html, I believe 3.11's behavior is wrong:

Formatted string literals, str.format(), and format() will use the mixed-in type’s __format__() unless __str__() or __format__() is overridden in the subclass

Your environment

• CPython versions tested on: 3.10.4 and 3.11.0b1
• Operating system and architecture: the docker images python:3.10-alpine and python:3.11-rc-alpine, on an x86_64 host