Build the docs with formats: [].

The build should succeed without problem and only HTML output has to be built. This can be easily checked by taking a look at the output commands from the Build itself and confirming that only one sphinx-build command was executed with the readthedocs builder.

>>> # Build at
>>> import datetime
>>> datetime.datetime.utcnow()  # UTC
datetime.datetime(2020, 1, 28, 18, 33, 3, 610545)