add support for ST7789V variant

[alangecker]

It allows you to use a ST7789V display by setting it as follows

disp = st7789.ST7789(
    [...]
    variant=st7789.VARIANT_ST7789V
)

all credits go to @mander1000, it is basically all their code from this commit https://github.com/pimoroni/st7789-python/commit/49f111bdb293ebd5d99f781bd827ceac044921b1, just separated out

closes #31