database name in URI is being ignored. If you wish to authenticate to {db name}, you must provide a username and password

When using a mongodb uri with a named db but no username or password eg `mongodb://localhost:27017/some_named_db` there's a warning from pymongo "database name in URI is being ignored. If you wish to authenticate to some_named_db, you must provide a username and password". This commit fixes that.

Additionally it removes the deprecated `pymongo.Connection()` and uses `pymongo.MongoClient()` instead.