Modernize the LDAP backend.

Review Request #2069 — Created Jan. 12, 2021 and submitted — Latest diff uploaded

Review Board
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.

Testing Done:
Unit tests pass for all versions of Python.

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