Chore: Unpin requirements

Chore: Unpin requirements

Having all requirements pinned was causing problems with dependency
resolution in pip. This worked fairly well in older versions, but
newer versions of pip are more strict in how they handle dependencies
of multiple packages. This was causing v0.31.1, the last version with
unpinned requirements, to be installed in cases where lftools was
installed alongside packages with conflicting requirements.

Issue: RELENG-3792
Change-Id: Ie8fb0dac45da5d3fba803ef66e216341516bc3ab
Signed-off-by: Eric Ball <eball@linuxfoundation.org>