__get_item__ needs to support different pixel orders

[kevinjwalters]

The get_time() function needs updating to deal with variable pixel ordering introduced in #15 #18.

    def __getitem__(self, index):
        if isinstance(index, slice):
            out = []
            for in_i in range(*index.indices(self._n)):
                out.append(
                    tuple(self._buf[in_i * 4 + (3 - i) + START_HEADER_SIZE] for i in range(3)))
            return out
        if index < 0:
            index += len(self)
        if index >= self._n or index < 0:
            raise IndexError
        offset = index * 4
        return tuple(self._buf[offset + (3 - i) + START_HEADER_SIZE]
                     for i in range(3))

I've not tested the code this is just from inspection after seeing the issue come up in adafruit/Adafruit_CircuitPython_WS2801#9. I'd imagine it'll end up similar to the neopixel implementation in https://github.com/adafruit/Adafruit_CircuitPython_NeoPixel/blob/163bd47a8385b993309a4b9100bb5956b2066a1d/neopixel.py#L182-L195

@RedAnon and @mcscope may be interested in this.

[kevinjwalters]

#42 looks like it changes this area of the code too