requirements: Pin jsonschema

The new 4.x release of jsonschema has borked our schemas for reasons I
do not yet fully understand (though I have an idea - see inline). Unbork
things now and fix our CI, while we work on a longer-term fix.

Signed-off-by: Stephen Finucane <stephen@that.guru>

requirements-test.txt

@@ -4,3 +4,9 @@ sqlparse~=0.4.0
 python-dateutil~=2.8.0
 tblib~=1.7.0
 openapi-core~=0.14.2
+# FIXME(stephenfin): We have to pin this to prevent a recurrence of [1]. It
+# seems openapi-core needs to gain support for OpenAPI 3.1 before we can fix
+# this properly
+#
+# [1] https://github.com/OAI/OpenAPI-Specification/issues/1368
+jsonschema<4.0