Add 'docs' tox environment

To make it easy to build docs with 'tox -e docs'.

Change-Id: Idd47c617196b25539b6b1fc88fac0a45b7fc0911
Use assertEqual instead of assertIs for strings

Checking identity of strings is not a great idea.
It might not work with different implementations. (eg. pypy)
Test usual equality instead.

Change-Id: Ib1a673a0ac116f2c80d066e94a8bd9a9ef8f518a
Merge "Add 'docs' tox environment"