Pin Jinja2 to 3.0.x for testing, and add upgrade note

docs/releases/2.16.2.md

@@ -13,3 +13,9 @@
  * Update django-treebeard dependency to 4.5.1 or above (Serafeim Papastefanos)
  * Fix permission error when sorting pages having page type restrictions (Thijs Kramer)
  * Allow bulk publishing of pages without revisions (Andy Chosak)
+
+## Upgrade considerations
+
+### Jinja2 compatibility
+
+Developers using Jinja2 templating should note that the template tags in this release (and earlier releases in the 2.15.x and 2.16.x series) are compatible with Jinja2 2.11.x and 3.0.x. Jinja2 2.11.x is unmaintained and requires ``markupsafe`` to be pinned to version ``<2.1`` to work; Jinja2 3.1.x has breaking changes and is not compatible. We therefore recommend that you use Jinja2 3.0.x, or 2.11.x with fully pinned dependencies.

setup.py

@@ -45,7 +45,7 @@ testing_extras = [
     'python-dateutil>=2.7',
     'pytz>=2014.7',
     'elasticsearch>=5.0,<6.0',
-    'Jinja2>=3.0,<4.0',
+    'Jinja2>=3.0,<3.1',
     'boto3>=1.16,<1.17',
     'freezegun>=0.3.8',
     'openpyxl>=2.6.4',