python: Strip leading and trailing spaces from docstrings.

This is enforced by Black ≥ 21.4b0.

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

zerver/lib/markdown/help_settings_links.py

@@ -96,7 +96,7 @@ settings_markdown = """
 
 class SettingHelpExtension(Extension):
     def extendMarkdown(self, md: Markdown) -> None:
-        """ Add SettingHelpExtension to the Markdown instance. """
+        """Add SettingHelpExtension to the Markdown instance."""
         md.registerExtension(self)
         md.preprocessors.register(Setting(), "setting", 515)