python: Convert percent formatting to Python 3.6 f-strings.

Generated by pyupgrade --py36-plus.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/openapi/python_examples.py

@@ -888,7 +888,7 @@ def upload_file(client: Client) -> None:
         "type": "stream",
         "to": "Denmark",
         "topic": "Castle",
-        "content": "Check out [this picture](%s) of my castle!" % (result['uri'],)
+        "content": "Check out [this picture]({}) of my castle!".format(result['uri'])
     })
     # {code_example|end}