Expand beyond Pro Trinket to other low-power 3.3V boards as slaves

[aufdenkampe]

Adafruit is deprecating the Pro Trinket!
Here's their waring:

Deprecation Warning: The Pro Trinket bit-bang USB technique it uses doesn't work as well as it did in 2014, many modern computers won't work well. So while we still carry the Pro Trinket so that people can maintain some older projets, we no longer recommend it. Please check out the Metro Mini (ATmega328 @ 5V 16 MHz), ItsyBitsy 32u4 5V 16MHz, ItsyBitsy 32u4 @ 3.3V 8MHz or ItsyBitsy M0 @ 3V 48MHz. All have built-in USB and are comparable in price! The ItsyBitsy's especially are about the same size and have native USB and tons of pins, so they're a very close compatible.

Given that we need it to be:

• 3.3V to be powered from a lipo battery powered loggers such as the EnviroDIY Mayfly, and
• low power itself because it always needs to be powered on,
  the only candidate listed by Adafruit is the ItsyBitsy 32u4 @ 3.3V 8MHz.

Are there other candidate boards?
What would it take to adapt them to function as an I2C counting device?

[aufdenkampe]

I noticed that @hawes1010 forked this repo to work for the Teensy with hawes1010@a0a62e8.

@hawes1010, let us know how that's been working!