Move our minimum Sphinx version to 1.7

As promised, drop support for some ancient sphinx releases, along with a
lot of the cruft that was required to make that support work.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs: kernellog.py: add support for info()

An extension may want to just inform about something. So, add
support for it.

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/0ddebd8677605d789d53433c8a5344c68da82a73.1604042072.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

doc: Cope with Sphinx logging deprecations

Recent versions of sphinx will emit messages like:

Documentation/sphinx/kerneldoc.py:103:
RemovedInSphinx20Warning: app.warning() is now deprecated.
Use sphinx.util.logging instead.

Switch to sphinx.util.logging to make this unsightly message go away.
Alas, that interface was only added in version 1.6, so we have to add a
version check to keep things working with older sphinxes.

Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>