Lib/
* Add support for X-ORIGIN in ldap.schema's ObjectClass
* Make initialize() pass extra keyword arguments to LDAPObject
* ldap.controls.sss: use str instead of basestring on Python 3
* Provide ldap._trace_* atributes in non-debug mode

Doc/
* Fix ReST syntax for links to set_option and get_option

Tests/
* Use intersphinx to link to Python documentation
* Correct type of some attribute values to bytes
* Use system-specific ENOTCONN value

Infrastructure:
* Add testing and document support for Python 3.7
* Add Python 3.8-dev to Tox and CI configuration
* Add Doc/requirements.txt for building on Read the Docs