This fixes up the LDAP backend to work correctly with Python 2 and 3. We

now initialize LDAP with bytes_mode=False, which will ensure that

python-ldap will use Unicode strings for all input and output on both

Python 2 and 3. It's the rough equivalent of a unicode_literals

import. That allows us to remove all the casting, which didn't work on

Python 3 anyway without emitting warnings.
Log output has been cleaned up, with some improved log output and a

dedicated logger.
The unit tests have been improved considerably. We now use spies to

track calls, rather than overriding the LDAPObject with a per-test

subclass. This allows us to ensure these overridden functions are

actually called, which was just an assumption we made before.
We didn't require LDAP support for development in the past, which meant

that some tests recently regressed. It's now a development-time

dependency. We also require a modern version for production.

Unit tests pass for all versions of Python.