Use Ellipsis to denote special/automatic syntax

[Barry Warsaw]

On python-ideas, Georg Brandl provided some nice syntax:

class Color(Enum):
    RED = 1
    GREEN = ... # becomes 2
    BLUE = ... # 3
    MAGENTA = 5
    FLUFL = ... # 6

class Color(FlagEnum):
    RED = 1
    GREEN = ... # becomes 2
    BLUE = ... # 4
    MAGENTA = 16
    FLUFL = ...

class Color(StringEnum):
    RED = ... # becomes 'red'
    GREEN = ... # etc.
    BLUE = ...