Refactor to f-strings to comply with pyupgrade

repo2docker/app.py

@@ -611,7 +611,7 @@ class Repo2Docker(Application):
             else:
                 # no port specified, pick a random one
                 container_port = host_port = str(self._get_free_port())
-                self.ports = {"%s/tcp" % container_port: host_port}
+                self.ports = {f"{container_port}/tcp": host_port}
             self.port = host_port
             # To use the option --NotebookApp.custom_display_url
             # make sure the base-notebook image is updated:

tests/unit/test_ports.py

@@ -96,7 +96,7 @@ def read_port_mapping_response(
         else:
             break
     else:
-        pytest.fail("Never succeeded in talking to %s" % url)
+        pytest.fail(f"Never succeeded in talking to {url}")
     assert "Directory listing" in r.text