Add warning about running build_usage on Python >3.4 (fixes #2123)

Python 3.6 added some new guaranteed hashing algorithms that will
show up as available in the docs even though the baseline for support
is Python 3.4; running build_usage with Python 3.4 will fix this.

# Conflicts:
#	docs/development.rst

docs/development.rst

@@ -297,7 +297,10 @@ Checklist:
 - update ``CHANGES.rst``, based on ``git log $PREVIOUS_RELEASE..``
 - check version number of upcoming release in ``CHANGES.rst``
 - verify that ``MANIFEST.in`` and ``setup.py`` are complete
-- ``python setup.py build_api ; python setup.py build_usage ; python setup.py build_man`` and commit
+- ``python setup.py build_api ; python setup.py build_usage ; python
+  setup.py build_man`` and commit (be sure to build with Python 3.4 as
+  Python 3.6 added `more guaranteed hashing algorithms
+  <https://github.com/borgbackup/borg/issues/2123>`_)
 - tag the release::
 
     git tag -s -m "tagged/signed release X.Y.Z" X.Y.Z