#4871: check that zipfile password is bytes, and give useful error message.

#4871: check that zipfile password is bytes, and give useful error message.

Previously passing a string in as the password would fail either with
an assertion error or a TypeError with a confusing error message.
Note that a string can't be accepted since zipfile has no way to
guess what encoding should be used to turn it into bytes.

Patch by Victor Stinner.