With the fix, parse_qs[l] doesn't handle bytes separators correctly.
There is an explicit type check for str/bytes:

    if not separator or (not isinstance(separator, (str, bytes))):
        raise ValueError("Separator must be of type string or bytes.")

but a bytes separator fails further down:

>>> import urllib.parse
>>> urllib.parse.parse_qs('a=1,b=2', separator=b',')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/pviktori/dev/cpython/Lib/urllib/parse.py", line 695, in parse_qs
    pairs = parse_qsl(qs, keep_blank_values, strict_parsing,
  File "/home/pviktori/dev/cpython/Lib/urllib/parse.py", line 748, in parse_qsl
    pairs = [s1 for s1 in qs.split(separator)]
TypeError: must be str or None, not bytes