Why is Python so hard to maintain

[RMJFlack]

BUT in Python 3.7, struct.pack is changed to byte type from string, and I'm having a real tough time trying to fix the code to with with it.

struct.pack() in 3.3.

struct.pack(fmt, v1, v2, ...)

Return a bytes object containing the values v1, v2, ... packed according to the format string fmt. The arguments must match the values required by the format exactly.

struct.pack() in 3.7.

struct.pack(format, v1, v2, ...)

Return a bytes object containing the values v1, v2, … packed according to the format string format. The arguments must match the values required by the format exactly.

So, in both 3.3 and 3.7 struct.pack() returns bytes object. There is no change according to official docs. Probably the problem is somewhere else in your code.

Hmmm ... you are right, It is IS 4 years since I looked at this code, and it was originally in 2.7. So Im not sure what the 3.3 version was about.
It seems to me the best way forward is to move to 3.7
I just need to make sure that the file created by struct.pack() in 3.7 (with bytes output) is the same as in 2.7 with string output.
Would this be the correct 'conversion' code:

def pack2string(byt):
    s = ""
    for b in byt:
        s += chr(b)
    return s

pixels += pack2string(struct.pack('<BBB',b,g,r))

I believe that in 2.7 each pixel formatted by struct.pack() is a triple of chr; in 3.3 it is a triple of int. Is that correct?