Build docs with Sphinx 3
The documentation builds started to fail due to PyTango intersphinx mapping problems. These appeared to be solved with the latests Sphinx. I made some effort to fix warnings after the Sphinx upgrade.
I take a profit of this PR to set default role in docs to obj. This way one could create cross-references (even intersphinx) with just back-quotes e.g.`taurus.Device` instead of :obj:`taurus.Device`
This is a house-keeping PR, so I won't bother you with the reviews and whenever it gets green in Travis I will auto-merge.
Initially it will fail - still waiting for a new version of docker image with the Sphinx v3. But I will retrigger later..