According to the documentation (https://docs.python.org/3/library/csv.html#writer-objects):

"""
A row must be an iterable of strings or numbers for Writer objects and a dictionary mapping fieldnames to strings or numbers (by passing them through str() first) for DictWriter objects.
"""

Byte object is neither string nor number. Passing it through str() gives your what you get.

Run Python with option -b or -bb to catch such kind of errors.