python: Avoid deprecated cgi module, removed in Python 3.13.

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

zerver/lib/markdown/__init__.py

@@ -1,6 +1,5 @@
 # Zulip's main Markdown implementation.  See docs/subsystems/markdown.md for
 # detailed documentation on our Markdown syntax.
-import cgi
 import html
 import logging
 import mimetypes
@@ -10,6 +9,7 @@ from collections import deque
 from collections.abc import Callable
 from dataclasses import dataclass
 from datetime import datetime, timezone
+from email.message import EmailMessage
 from functools import lru_cache
 from re import Match, Pattern
 from typing import Any, Generic, Optional, TypeAlias, TypedDict, TypeVar, cast
@@ -441,7 +441,9 @@ def fetch_open_graph_image(url: str) -> dict[str, Any] | None:
             if res.status_code != requests.codes.ok:
                 return None
 
-            mimetype, options = cgi.parse_header(res.headers["Content-Type"])
+            m = EmailMessage()
+            m["Content-Type"] = res.headers.get("Content-Type")
+            mimetype = m.get_content_type()
             if mimetype not in ("text/html", "application/xhtml+xml"):
                 return None
             html = mimetype == "text/html"