Make name and value read-only for Enums

[AlexWaygood]

These attributes are read-only properties at runtime; they should be read-only properties in the stub, also.

The following snippet currently fails at runtime, but passes mypy:

from enum import Enum

class Menu(Enum):
    SPAM = 1
    HAM = 2

Colours = Enum('Colours', 'RED YELLOW BLUE')

Menu.SPAM.name = 'BACON'
Menu.HAM.value = 4
Colours.RED.name = 'PURPLE'
Colours.YELLOW.value = 5

With this proposed patch applied, mypy yields the following true-positive errors for the above snippet:

test_enum.py:9: error: Property "name" defined in "Enum" is read-only
test_enum.py:10: error: Property "value" defined in "Enum" is read-only
test_enum.py:11: error: Property "name" defined in "Enum" is read-only
test_enum.py:12: error: Property "value" defined in "Enum" is read-only

[AlexWaygood]

The Enum class does some magic to ensure that the name and value properties result in AttributeErrors if you try to access them directly from the class, rather than from a member of the class. I suspect that's what is throwing off stubtest. They exist, I promise!

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

ignite (https://github.com/pytorch/ignite)
+ ignite/engine/events.py:141: error: unused "type: ignore" comment

steam.py (https://github.com/Gobot1234/steam.py)
+ steam/enums.py:188: error: Definition of "name" in base class "Enum" is incompatible with definition in base class "Enum"  [misc]

[AlexWaygood]

A follow-up PR is required -- I didn't realise that name and value are overriden in the stub for IntEnum and Flag! I'll get on it now.

[AlexWaygood]

A follow-up PR is required -- I didn't realise that name and value are overriden in the stub for IntEnum and Flag! I'll get on it now.

#6578 is the follow-up.

[AlexWaygood]

A follow-up PR is required -- I didn't realise that name and value are overriden in the stub for IntEnum and Flag! I'll get on it now.

#6578 is the follow-up.