Fix trailing slash

Getting the following when verifying JWT in IDhub (possibly first service doing this?)

INFO:apigatewayauth.authentication:Incoming API token failed verification: Token has wrong audience https://webapp-{foobar}-nw.a.run.app, expected one of ['https://webapp-{foobar}-nw.a.run.app/']

Could override expected audience via env var but better the default behaves the same way as the API Gateway creates the JWT.

Part of https://gitlab.developers.cam.ac.uk/uis/devops/iam/idhub/api/-/issues/90

apigatewayauth/tests/test_apigateway_auth.py

@@ -246,8 +246,8 @@ class APIGatewayAuthTestCase(TestCase):
         self.assertEqual(self.mock_verify_token.call_args[0][0], self.expected_token)
         # Both audience with and without trailing slash should be passed to the token verification
         self.assertEqual(
-            self.mock_verify_token.call_args[1]["audience"],
-            ["https://audience.invalid/", "https://audience.invalid"],
+            set(self.mock_verify_token.call_args[1]["audience"]),
+            {"https://audience.invalid/", "https://audience.invalid"},
         )
         self.assertEqual(
             self.mock_verify_token.call_args[1]["certs_url"], "https://issuer.invalid/certs"