Stop generating Python 2-style enums

[tjprescott]

from enum import Enum
from six import with_metaclass
from azure.core import CaseInsensitiveEnumMeta

class Answers(with_metaclass(CaseInsensitiveEnumMeta, str, Enum)):
    NONE = "none"
    EXTRACTIVE = "extractive"

If you try and run pylint on this, it will crash with an error that "Const is not iterable". (See: pylint-dev/pylint#5935)

Even if pylint fixes the error, we are pinned to pylint 2.5.2 and this still happens there.

This does not affect our SDKs much because most of these enums live in the _generated folder, which is skipped by our pylint rcfile. However, if someone follows our generated code and creates a manual enum, they will observe this behavior. Since we no longer support Python 2, we should start generating these enums with the Python 3 syntax, which is pylint-compatible:

from enum import Enum
from azure.core import CaseInsensitiveEnumMeta

class Answers(str, Enum, metaclass=CaseInsensitiveEnumMeta):
    NONE = "none"
    EXTRACTIVE = "extractive"

cc/ @lmazuel @l0lawrence @kashifkhan

[iscai-msft]

fixed by #1202 and released in 5.15.0