mypy: Enable new error explicit-override.

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

zerver/lib/url_preview/parsers/open_graph.py

@@ -1,11 +1,14 @@
 from urllib.parse import urlparse
 
+from typing_extensions import override
+
 from zerver.lib.url_preview.types import UrlEmbedData
 
 from .base import BaseParser
 
 
 class OpenGraphParser(BaseParser):
+    @override
     def extract_data(self) -> UrlEmbedData:
         meta = self._soup.findAll("meta")