ec06206754
Sphinx has disallowed the practice of setting language = None [0],
which is generating a warning that the docs job interprets as an
error. Stop this by setting the language to 'en', which is the
Sphinx default.
[0] 3596590317/sphinx/config.py (L183)
Change-Id: Ibd71783419a3eb0620e15796087e46e936964ae2