SPHINXOPTS commented and sphinx build upgrade to 3.1.2

[Niehaus]

Are you working on some issue? Identify the issue!

Working on #595

📑 Description of the Change

By commenting the line 5 of docs Makefile SPHINXOPTS = -W you've got those warnings, that leads
to remove the inheritance of metaclass=MetaBitMask from GenericBitMask, what reduces significantly
the amount of warnings. Maybe this could mean that the problem is in the meta class MetaBitMask.
Also changed sphinx version in dev.txt.

[hdiogenes]

For some reason Scrutinizer is having problems with the required versions, maybe because it uses an old version of pip. Can you reproduce that on your machine, @Niehaus? It doesn't make a lot of sense to me, since kytos-sphinx-theme 0.0.8 seems to have the newer sphinx as a requirement, not 2.1.

The conflict is caused by:
    The user requested sphinx==3.1.2
    kytos-sphinx-theme 0.0.8 depends on sphinx<2.1