HTTP basic auth: encode username and password as UTF-8 (#315)

As discussed upstream in
https://github.com/psf/requests/issues/4564

 , HTTP basic auth
usernames and passwords sent to requests as Python text strings
are encoded as latin1. This of course makes it impossible to
log in with a username or password containing characters not
represented in latin1, as the reporter of #315 found out.

To work around this rather old-fashioned default, let's intercept
string usernames and passwords and encode them as utf-8 before
sending them to requests.

Anyone dealing with a really old server that can't handle utf-8,
or something like that, can encode the username and password
appropriately and provide them as bytestrings.

Signed-off-by: Adam Williamson <awilliam@redhat.com>