The POST data should be bytes. So in the attached test case, instead of

request = urllib.request.Request('http://does.not.matter', 'foo=bar')

it should be:

request = urllib.request.Request('http://does.not.matter', b'foo=bar')

And the Content-Length will be calculated using this logic.

mv = memoryview(data)
Content-length = len(mv) * mv.itemsize

Should we emphasize further that data should be bytes? I think error
message can be improved here.  This issue is not a blocker.