Add Adafruit-Blinka to requirements

[evaherrada]

No description provided.

[makermelissa]

@kattni and @tannewt, I thought I remembered there was a reason this wasn't added before. Do you remember if it was because this was a special library? If we are not adding it, we should probably add some kind of exception.

[tannewt]

@makermelissa Blinka isn't needed because the library doesn't import any CircuitPython specific modules.