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/lib/bugdown/tabbed_sections.py

@@ -124,7 +124,7 @@ class TabbedSectionsPreprocessor(Preprocessor):
                 # Wrapping the content in two newlines is necessary here.
                 # If we don't do this, the inner Markdown does not get
                 # rendered properly.
-                content='\n{}\n'.format(content))
+                content=f'\n{content}\n')
             tab_content_blocks.append(tab_content_block)
         return '\n'.join(tab_content_blocks)