Pin Jinja to below 3.1.x

We can’t upgrade to Jinja 3.1.0 because the `escape` module has been
moved to the `markupsafe` library. The old version of Flask we are
using tries to import `escape` from `jinja2`, causing an error.

See https://jinja.palletsprojects.com/en/3.1.x/changes/#version-3-1-0

requirements.in

@@ -9,7 +9,7 @@ Flask-WTF==1.0.1
 wtforms==3.0.1
 Flask-Login==0.6.1
 werkzeug==2.1.2
-jinja2==3.1.2
+jinja2==3.0.3  # Can’t be upgraded until we are using Flask >= 2
 
 blinker==1.4
 pyexcel==0.7.0

requirements.txt

@@ -91,7 +91,7 @@ itsdangerous==1.1.0
     #   flask
     #   flask-wtf
     #   notifications-utils
-jinja2==3.1.2
+jinja2==3.0.3
     # via
     #   -r requirements.in
     #   flask