Migrate RCS to use Python v3

[james-rae]

Update rcs to use Python v3.6.4. The main impetus for the upgrade is to gain access to enhanced https/ssl support. It's also generally good to upgrade to a more modern version (v3.0.0 came out in late 2008; v2.0.1 was in 2001).

Most of the python source code in rcs should be compatible with v3.x. There will be a few changes. A useful reference point is changes made for CIP project in this commit (private repo, sorry outside viewers). It's a large initial commit but contains updated python files.

Primary tasks

• installing / updating python on rcs hosting server
• update .travis.yml
• investigating dependencies in requirements.txt and requirements-dev.txt, updating versions if appropriate.
• a real thorough amount of testing
• updating docs
  • readme.rst
  • docs/conf.py
  • docs/dev/setup.rst
  • docs/admin/deployment.rst
• argue about new rcs version number and update it