assert keys is tuple object in BitHandler init method

[ruandao]

django-bitfield/bitfield/types.py

Line 121 in 32d5f06

# TODO: change to bitarray?

may be need a method to convert key to value, i try many time to find out how to convert

        result = BitHandler(0, tuple(TeamPerm.permission))
        for label in data:
            setattr(result, label, True)