Bug 1926: Update version of Six
The IM now uses six 1.16.0, so the requirements-dev.txt needs to be
updated to match
Additional dependencies updated:
lxml
pyang
idna
requests
Fixes bug 1926
Change-Id: I7d8f9ca9a0fd5e0d0e2bdb1900ef247a5b41fccc
Signed-off-by: beierlm <mark.beierl@canonical.com>