test_reading_color_palette: specify colormode

[keturn]

This test was failing locally for me:

>       assert str(colorful.myBlack) == '\033[30m'
E       AssertionError: assert '\x1b[38;2;0;0;0m' == '\x1b[30m'

I assume because the colormode it detected here was different than the colormode when the assertion was written.

It had a skip condition on it to require a tty, but I'm guessing that was a copy-paste and terminal detection was not supposed to be part of the palette-reading test.

[timofurrer]

Thanks 🎉