BUG: fix missing/invalid imports.
ENH: don't try to guess when to use EPD_auth vs EPD_username anymore.

When creating a Configuration instance, we now detect whether we should
use keyring or not, and behave accordingly instead of second-guessing
from EPD_auth/EPD_username values.

Next step is to get away from EPD_auth/EPD_username completely (except
for backward compat).
REF: remove EPD_auth/EPD_username from internal config state.

We are still handling it when parsing config file of course, and expose
them as properties for similar reasons. We may want to make those
properies read-only, though.