fix mistaken use of PWM channel for slice

[dhalbert]

• Fixes Crash when using countio and pwmio together. #5556. May also fix some mysterious other PWM problems.

In RP2040 PWM, the term channel is overloaded. Sometimes it means one of two channels (A or B) in a slice, sometimes it means one of 16 PWM output channels, and sometimes it is another name for slice (of which there are 8).

It appears we used an AB channel number when we meant to use a slice number. This caused #5556.

I renamed some uses of the word channel, when it meant an A or B slice channel. I renamed those channels to ab_channel, to make this clearer. But I'd kind of like another term for the 16-instance channels as well, maybe pwm_channel, or something.

I also made a doc issue for the RP2040 folks about the overloading: raspberrypi/pico-feedback#209

[dhalbert]

black just got updated, and I had to fix the formatting on a couple of test .py files.

[tannewt]

Thank you! Agreed that the term "channel" is overloaded.