Python 3 Support

[Peter Russell]

When importing Localmail on Python 3.6.4 on Windows 10, I see the following stack trace. It appears that the imports in cred.py should be re-written to use relative imports.

---------------------------------------------------------------------------
ModuleNotFoundError Traceback (most recent call last)
<ipython-input-32-661df2d30865> in <module>()
----> 1 import localmail

~\.virtualenvs\Signatures-lUlE5qBd\lib\site-packages\localmail\__init__.py in <module>()
     17 from twisted.internet import reactor
     18 from twisted.cred import portal, checkers
---> 19 from .cred import TestServerRealm, CredentialsNonChecker
     20 from .smtp import TestServerESMTPFactory
     21 from .imap import TestServerIMAPFactory

~\.virtualenvs\Signatures-lUlE5qBd\lib\site-packages\localmail\cred.py in <module>()
     19 from twisted.mail import smtp, imap4
     20
---> 21 from imap import IMAPUserAccount
     22 from smtp import MemoryDelivery
     23

ModuleNotFoundError: No module named 'imap'

[Peter Russell (qwertyface)]

Actually there's a bit more work to support Python 3 than just fixing the imports.