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

RonnyPfannschmidt/borg,raxenak/borg,RonnyPfannschmidt/borg,edgimar/borg,edgimar/borg,RonnyPfannschmidt/borg,edgewood/borg,RonnyPfannschmidt/borg,raxenak/borg,raxenak/borg,edgewood/borg,RonnyPfannschmidt/borg,edgewood/borg,edgimar/borg,edgimar/borg,raxenak/borg,edgewood/borg