Simplified UART has wrong byte format: int8 instead of uint8

[antonvh]

After this commit, PUPDevice.write(1, (-127, -128, -126, -1, 127, 128, 129, 130) ) results in 7F, 7F, 81, FF, 7F, 7F, 7F

pybricks/pybricks-micropython@1ae92c3

So signed bytes become unsigned, every out-of-range number becomes 7F, and the number 128 is unreachable.

The code should probably have uint8 everywhere.

[antonvh]

Expected results for PUPDevice.write(1, (-129, -128, -127, -1, 127, 128, 129, 130) ) Two options.

1. preferably uint8, this leads to the least byte conversion. The protocol here only mentions bytes, no int8: https://github.com/pybricks/technical-info/blob/master/uart-protocol.md
   ('0x00', '0x00', '0x00', '0x00', '0x7f', '0x80', '0x81', '0x82')

2. alternatively int8, as is. BUT with (a) -128 reachable, (b) clamping down for numbers below -128, so -129 becomes -128 (0x80) and (c) raw bytes writing option.
   ['0x80', '0x80', '0x82', '0xff', '0x7f', '0x7f', '0x7f', '0x7f']

raw bytes:
PUPDevice.write(1, b'\x00\x01\x02' )