six: use latest version of six
pocount: better counting of things that looks like XML

Copied the XML re from placeables.  Probably should be sharing this re.
pocount: bump version for e2587ba8

Since this potentially changes counts we should force users to
invalidate any caches.
Tests: fixed docstring
properties: use proper Sphinx deprecations docs
pypo: use depr. decorator, add depr. reason

Add the reason for the deprecation and for keeping the function around.
Taken from the original commit message.
styleguide: documenting deprecation of features