build: kludge around Sphinx + Jinja2 + MarkupSafe requirement conflicts

Don't want to mess with Sopel 7.x's code at this point unless I HAVE to.
This is exactly the kind of stupid little thing that's only an issue for
us developers, anyway, so it's not like it needs a bugfix release.

netlify-build.sh

@@ -13,7 +13,9 @@ echo "Generating plugin command/config pages"
 python3 document_sopel_plugins.py --sopel=_sopel
 
 echo "Installing Sopel's dev dependencies"
-pip3 install -r ./_sopel/dev-requirements.txt
+# Jinja2 pin for dependency hell on Sopel 7.x;
+# shouldn't be necessary once we switch to docs for 8.0
+pip3 install -r ./_sopel/dev-requirements.txt 'Jinja2<3.1'
 
 echo "Building Sphinx docs"
 cd _sopel/docs