docs: sphinx/requirements: Limit jinja2<3.1

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>

Documentation/sphinx/requirements.txt

@@ -1,2 +1,4 @@
+# jinja2>=3.1 is not compatible with Sphinx<4.0
+jinja2<3.1
 sphinx_rtd_theme
 Sphinx==2.4.4