docs: sphinx/requirements: Limit jinja2<3.1

docs: sphinx/requirements: Limit jinja2<3.1

BugLink: https://bugs.launchpad.net/bugs/1969110
commit be78837ca3c88eebd405103a7a2ce891c466b0db upstream.

jinja2 release 3.1.0 (March 24, 2022) broke Sphinx<4.0.
This looks like the result of deprecating Python 3.6.
It has been tested against Sphinx 4.3.0 and later.

Setting an upper limit of <3.1 to junja2 can unbreak Sphinx<4.0
including Sphinx 2.4.4.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: stable@vger.kernel.org # v5.15+
Link: https://lore.kernel.org/r/7dbff8a0-f4ff-34a0-71c7-1987baf471f9@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit f1d93653715561f17b49e0fb5ed953660fecd115)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>