Defining _order_ in an enum class rejected

[JukkaL]

This generates an error, even though it's valid:

from enum import Enum

class E(Enum):
    _order_ = 'X Y'  # Cannot override writable attribute "_order_" with a final one
    X = 1
    Y = 2

According to the docs, _order_ is a somewhat magical attribute, but it's okay to define it.

The error started happening as a side effect of #10852.

It looks like _order_ actually isn't a valid attribute at runtime, so a potential fix would be to remove it from typeshed. If we want to do this right, we should also remove an explicit definition from the TypeInfo, since it's not available at runtime.

@sobolevn What do you think?

This is a regression, so marking as high priority.

[sobolevn]

I think that Enum subclasses should be special cased for Cannot override writable attribute "_order_" with a final one rule. Because, a lot of things like

from enum import Enum

class E(Enum):
    value = 1  # E: Cannot override writable attribute "value" with a final one

print(E.value)

work just fine in runtime, but mypy is not really happy.

[sobolevn]

My proposal: #11713