timeout: Rename to unsafe_timeout.

This timeout strategy using asynchronous exceptions has a number of
safety caveats (read the docstring!!) and should only be used in very
specific circumstances.

Signed-off-by: Anders Kaseorg <anders@zulip.com>
(cherry picked from commit 72018cc26b)

zerver/lib/test_classes.py

@@ -1543,7 +1543,7 @@ Output:
         markdown.__init__.do_convert.
         """
         with mock.patch(
-            "zerver.lib.markdown.timeout", side_effect=subprocess.CalledProcessError(1, [])
+            "zerver.lib.markdown.unsafe_timeout", side_effect=subprocess.CalledProcessError(1, [])
         ), self.assertLogs(level="ERROR"):  # For markdown_logger.exception
             yield