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>