[CI/DevOps] Update Docker/CI configs to use Python3

[de-sh]

The tests are failing on python3 branch as we are trying to run python3 programs on a python2.7 system. I have updated the Dockerfile and .travis.yml to correct this error.

Some updates are still required, needs discussion

[de-sh]

Should we include 3.5, 3.5-dev, 3.6, 3.6-dev and 3.7-dev as well?

[tabatkins]

The tests are failing because I'm not done fixing errors and rebasing them, but yes, this is a necessary change as well. 😀

[de-sh]

  File "/bikeshed-master/bikeshed/config/dfnTypes.py", line 50, in <module>
    dfnTypes = frozenset(dfnClassToType.values() + ["dfn"])
TypeError: unsupported operand type(s) for +: 'dict_values' and 'list'
The command '/bin/sh -c bikeshed update' returned a non-zero code: 1

This is an error I am trying to solve with the docker instance...

EDIT: How silly of me to be using python3 on master. It was there within the Dockerfile itself! 🤦‍♂️ 😂

Seems like it needs to be rewritten.