Update enum for Python 3.6

[brettcannon]

Bunch of changes to the enum module in Python 3.6. The type hints are also missing things that pre-date 3.6 like __members__. (The ability to iterate is covered by python/mypy/issues/2305.)

[JelleZijlstra]

A bunch of new things were already added in bbc7594 and Guido seems to be working on the metaclass in bebb49a. I added some missing methods in 43c48cc and I can add any missing functionality on the metaclass once Guido's commit lands.

[FichteFoll]

I still get this with mypy 0.521 and Python 3.6.2:

import enum


class NetworkEventName(str, enum.Enum):
    CONNECTED = "connected"
    DISCONNECTED = "disconnected"
    CLOSE_REQUEST = "close_request"
    MESSAGE = "message"
    RAW_LINE = "raw_line"


print(NetworkEventName.__members__)

> mypy /tmp/enum_test.py
/tmp/enum_test.py:12: error: Type[NetworkEventName] has no attribute "__members__"

> python /tmp/enum_test.py
OrderedDict([('CONNECTED', <NetworkEventName.CONNECTED: 'connected'>), ('DISCONNECTED', <NetworkEventName.DISCONNECTED: 'disconnected'>), ('CLOSE_REQUEST', <NetworkEventName.CLOSE_REQUEST: 'close_request'>), ('MESSAGE', <NetworkEventName.MESSAGE: 'message'>), ('RAW_LINE', <NetworkEventName.RAW_LINE: 'raw_line'>)])

[gvanrossum]

Can someone please file a separate issue for that?

[gvanrossum]

Also may be it's easy to fix with a PR to typeshed?

[FichteFoll]

Created as #1590. The __members__ property is defined, so I have no idea why it wouldn't work tbh.

typeshed/stdlib/3.4/enum.pyi

Lines 12 to 13 in 385b9c8

	@property
	def __members__(self: Type[_T]) -> Mapping[str, _T]: ...