Proposed commit log message:
```
[NEW PORT]: mail/managesieve3: Pure Python client for managing Sieve scripts
A pure Python client implementation of "A Protocol for Remotely
Managing Sieve Scripts", as defined in RFC-5804.
WWW: https://bitbucket.org/ericvsmith/managesieve3
Reviewed_by: koobs
Approved by: koobs (mentor)
Differential_Revision:
```