HTTPDigestAuth fails on non-latin credentials

There was issue reported, which is closed with bad results.

https://github.com/psf/requests/blob/4f6c0187150af09d085c03096504934eb91c7a9e/requests/auth.py#L59-L63

Don't pass unicode strings in the arguments, but use UTF8 bytes instead.

self.session.get(main_url, auth=requests.auth.HTTPDigestAuth("Сергей_Ласточкин".encode('UTF-8'), '1234'))

Originally posted by @D-stefaang in https://github.com/psf/requests/issues/5089#issuecomment-763569911

But this is wrong! When i try to set user 'Ondřej' with this advice, requests send bad string:

HTTPDigestAuth('Ondřej'.encode('utf-8'), 'heslíčko')

creates header starts with wrong username!

Digest username="b'Ond\xc5\x99ej'"