#13700: Make imap.authenticate with authobject work.

This fixes a bytes/string confusion in the API which prevented
custom authobjects from working at all.

Original patch by Erno Tukia.
Merge: #13700: Make imap.authenticate with authobject work.

This fixes a bytes/string confusion in the API which prevented
custom authobjects from working at all.

Original patch by Erno Tukia.
#17143: fix buildbot failures on Windows.