python: Convert "".format to Python 3.6 f-strings.

Generated by pyupgrade --py36-plus --keep-percent-format, but with the
NamedTuple changes reverted (see commit
ba7906a3c6, #15132).

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

zerver/webhooks/sentry/view.py

@@ -78,7 +78,7 @@ def convert_lines_to_traceback_string(lines: Optional[List[str]]) -> str:
             if (line == ""):
                 traceback += "\n"
             else:
-                traceback += "     {}\n".format(line)
+                traceback += f"     {line}\n"
     return traceback