Make the Python trees and sets accept an explicit None argument to indicate an open range in keys(), values() and min/maxKey(), as per the interface documentation and the C implementation. Except actually, the C implementation didn't accept a None for mim/maxKey, so make it actually do that.

Items must be in reversed-sorted order.

Fixes #2 (maybe).

Previously, the extra required the 'ZODB3' metapackage:  depending on the
version used, the metapackage could pull in stale versions of **this**
package and 'persistent'.

I don't think we intentionally checked for Python 4.

Broeken version could contain garbage characters.

- Denominate files to indicate ReST.

- Drop .bzrignore.

Drop no-longer-used 'include'.

Note that the Python version exists only for BBB:  no application code
should instantiate one directly.

We need to set up an interpreteer copy which has 'persistent' already
installed, to prevent the on-demand eggs for 'persistent' and
'zope.interface' from screwing up the rest of the buildout.