A colleague wanted to use the 'c' format code in the struct module. I think I've implemented it correctly, and tested both pack() and unpack(). Under Windows, Python 3.5.2 throws an exception when passing a one-byte string as a parameter to struct.pack(), but in MicroPython, the mp_buffer_info_t looks the same for both b'C' and 'C'.

Also, I should be throwing a struct.error exception, but that hasn't been implemented elsewhere in the module yet, so I just fell back on a ValueError.

>>> struct.unpack('<HBB5c', b'\x01\x00\x02\x03ABCD')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: buffer too small
>>> struct.unpack('<HBB5c', b'\x01\x00\x02\x03ABCDE')
(1, 2, 3, b'A', b'B', b'C', b'D', b'E')
>>> struct.unpack('<HBB5c', b'\x01\x00\x02\x03ABCDEF')
(1, 2, 3, b'A', b'B', b'C', b'D', b'E')
>>> struct.pack('<HBB5c', 1, 2, 3, b'A', b'B', b'C', b'D', b'E')
b'\x01\x00\x02\x03ABCDE'
>>> struct.pack('<HBB5c', 1, 2, 3, b'A', b'B', 'C', b'D', b'E')
b'\x01\x00\x02\x03ABCDE'
>>> struct.pack('<HBB5c', 1, 2, 3, b'A', b'BX', b'C', b'D', b'E')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: char format requires a bytes object of length 1