default to collapsing pre-releases in sphinxext
default to collapsing pre-releases in sphinxext

Change-Id: I2e7b3af24cff19c60e933cd7766c438aa76ed1b8
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

openstack/reno