Enhancement: sending data in chunks

[qbolsee]

Hi,

When using pyupdi to flash a ATtiny1614 using a SAMD11C, I experienced some very specific bug forcing me to send data in chunks smaller than 64 bytes. I documented the issue here:
http://fabacademy.org/2020/labs/ulb/students/quentin-bolsee/projects/d11c_ftdi-updi/#arduino-core

I propose the following patch to handle this kind of issue:
chunk_patch.txt

Basically, the send() function in physical.py is modified to slice up data in chunks of a given max size. I think it's reasonable to use chunks when waiting for the echo, otherwise you can easily fill up some buffers on the USB serial, leading to strange behaviors.