Deprecate getitem access by enum value name

[Barry Warsaw]

MyEnum[foo] can be used to access enum values by value or name. This is ambiguous:

class Fruit(Enum):
    apple = 'red'
    banana = 'yellow'
    tangerine = 'orange'
    orange = 'reddish yellow'

So what should Fruit['orange'] return?

Nick pointed out that we can use getattr() to access to get values by name, e.g.

getattr(Fruit, Fruit.orange.name) or getattr(Fruit, 'orange')

We eventually want to remove this, so now we'll deprecate it.