bpo-38659: [Enum] add _simple_enum decorator (GH-25497)

add:

* `_simple_enum` decorator to transform a normal class into an enum
* `_test_simple_enum` function to compare
* `_old_convert_` to enable checking `_convert_` generated enums

`_simple_enum` takes a normal class and converts it into an enum:

    @simple_enum(Enum)
    class Color:
        RED = 1
        GREEN = 2
        BLUE = 3

`_old_convert_` works much like` _convert_` does, using the original logic:

    # in a test file
    import socket, enum
    CheckedAddressFamily = enum._old_convert_(
            enum.IntEnum, 'AddressFamily', 'socket',
            lambda C: C.isupper() and C.startswith('AF_'),
            source=_socket,
            )

`_test_simple_enum` takes a traditional enum and a simple enum and
compares the two:

    # in the REPL or the same module as Color
    class CheckedColor(Enum):
        RED = 1
        GREEN = 2
        BLUE = 3

    _test_simple_enum(CheckedColor, Color)

    _test_simple_enum(CheckedAddressFamily, socket.AddressFamily)

Any important differences will raise a TypeError