We need 0.25.0 for py3 support, and increasing it for both is simplest.
Also drag forward dnspython, as newer eventlet requires a newer
dnspython. Note that we still don't explicitly list it as a dependency
for py3; rather, it's been getting installed as a requirement for
eventlet.
Change-Id: If933b713bdd1dfc348959771d60c0756a9a94477