auth: cleanup of EXTERN_TYPE_INTERNAL Don't set it in top level namespace - it is a weak link between the database and the actual implementation. Don't make it more than that. Don't hardcode in that many places that 'internal' is the default - just call it DEFAULT_AUTH_TYPE. Don't use it for extern_name - it is only intended for use as extern_type. Remove unused uses.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.