always use sitepackages

we really don't need to be compiling lxml at 3.5 when 2.3 is good
enough. So specify that we can always use sitepackages, as well
as don't pip -U, so we use requirements as intended.

Change-Id: If9f337d6bb688d46ab68dc1276e0f499a7dec8fd
Merge "always use sitepackages"