Pin Jinja to <3.1.0 (#12297)

as 3.1.0 removed the deprecated jinja2.Markup class which we still rely on.

changelog.d/12297.misc

+Pin Jinja to <3.1.0, as Synapse fails to start with Jinja 3.1.0.
\ No newline at end of file

synapse/python_dependencies.py

@@ -74,7 +74,8 @@ REQUIREMENTS = [
     # Note: 21.1.0 broke `/sync`, see #9936
     "attrs>=19.2.0,!=21.1.0",
     "netaddr>=0.7.18",
-    "Jinja2>=2.9",
+    # Jinja2 3.1.0 removes the deprecated jinja2.Markup class, which we rely on.
+    "Jinja2<3.1.0",
     "bleach>=1.4.3",
     # We use `ParamSpec`, which was added in `typing-extensions` 3.10.0.0.
     "typing-extensions>=3.10.0",