Plugin requires Python 3

[jwilliams-shadowsoft]

Although it is not completely clear why, it seems that this plugin requires Python 3 to be able to authenticate against a Mongo 3.6 database that requires authentication. (This may be related to the versions of pymongo that are available under Python 2).

Every time we tried the plugin using Python 2.7 it would fail authentication, but we would not get an "authentication failed" error, we would get an error stating that we did not have enough permission to perform the operation (except for connection check, of course).

Please change the documentation to show that Python 3 is a requirement.