Convert to ldap3

[thatarchguy]

This will help get us on python 3

#95 (comment)

[mprahl]

I'm working on this and my branch is at:
https://github.com/StackFocus/PostMaster/tree/ldap3-migration

I am having problems with the unit tests though as explained here:
cannatag/ldap3#240

Edit:
The owner of LDAP3 fixed the issue in the dev branch. Just need to wait for a release.

Edit2:
I ran into another issue. I submitted a PR here:
cannatag/ldap3#241

Edit3:
He ended up writing it an alternative way and now it's working. A new version was just released.

[mprahl]

#179 addresses this