Add documentation for Length class

Conflicts:
	CHANGES.rst
	setup.py

I tried to tighten the whitelist, but that didn't work.

distutils must die.

Work around AttributeError raised by Python 3.5.x.

Alternative to PR #55.

* Use macpython everywhere.

Add 3.6 for mac.

See zopefoundation/persistent#59

* Manually install persistent for travis.

* plain pypy3 is python 3.2 which won't run newer setuptools

Add support for Python 3.6

Allow None as a special key (sorted smaller than all others).

Cuz the complicated version was wrong, in addition to being complicated :)

Also removed s stupid non-optimization in compare

Python/C OO consistency

Check key identity before ==. This matches the C implementation.
Fixes #50

Add a pickle test to the work done in #54. Fixes #51. Fixes #53 (for
Python 2).

Check for object key default comparison on set only

Point the doc link to the rendered site.

Mostly for PyPI benefit.

[skip ci]

Add requirements file for RTD.

More consistency between Python and C impls.

Fixes #42.

See CHANGES.rst for details on what changes. Additional test cases cover
the changed behaviour.

build BTrees for 3.4