Type `_order_` field in `Enum`

Right now this code passes type checking:

from enum import Enum

class E(Enum):
    _order_ = 'X Y Z'  # Should be 'X Y'
    X = 1
    Y = 2

But, in runtime it fails:

» python out/ex.py       
Traceback (most recent call last):
  File "/Users/sobolev/Desktop/mypy/out/ex.py", line 3, in <module>
    class E(Enum):
  File "/Users/sobolev/.pyenv/versions/3.10.0/lib/python3.10/enum.py", line 348, in __new__
    raise TypeError('member order does not match _order_')
TypeError: member order does not match _order_

We need to check that _order_ field is valid.
This can be a great addition to my "better Enums" series of PRs.

Start by tracing mypy's handling of Python Enum class bodies and reproduce the issue's example with the mismatched _order_ value. The work is done when type checking reports an invalid member order while accepting a matching order, with focused tests covering both cases.