Docs build was broken with:
TypeError: __init__() got an unexpected keyword argument 'handle_io_errors'
Upgrade sphinx requirements to those of a recent decade, and replace
the oslosphinx extension and requirement with openstackdocstheme.
At the same time, split the deps for the docs build out of
test-requirements.txt and put them in docs/requiremenst.txt as the gods
intended.
Change-Id: I6f82b5c37217ca2bdcf85c1ce07293b3d1026370
(cherry picked from commit 0d5042b0ce)
8fb5ba3dd1