Use latin1 encoding to decode bytes

[radarhere]

Resolves #5865

That issue reports an error decoding bytes. A self-contained example of that error is

>>> bytes(list(range(46))+[231, 1]).decode()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe7 in position 46: invalid continuation byte

It can be fixed by using "latin1"

>>> bytes(list(range(46))+[231, 1]).decode("latin1")
'\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f !"#$%&\'()*+,-ç\x01'

"latin1" is already used in two other places in setup.py -

Pillow/setup.py

Line 412 in c5d9223

.decode("latin1")

and

Pillow/setup.py

Line 528 in c5d9223

.decode("latin1")