What to change on stem release 1.7

When legacy/trac#26914 (closed) is fixed:

• when stem 1.7 is in pypi:
  • change setup.py to add stem >= 1.7 dependency and remove dependency links
  • change tox.ini to remove install_command and use tox default
• when stem 1.7 is in Debian sid
  • change INSTALL.md to recommend installing from Debian
  • update Debian source to:
    • remove the patch
    • change stem dependency to 1.7
• when stem 1.7 is in Debian stretch-backports
  • create sbws package for Debian stretch-backports