bugdown: Revert NamedTuple declarations to old style.
bugdown: Revert NamedTuple declarations to old style.

This reverts part of commit 8bcdf4ca970dd5cdf532992a489a21565303fd10
(#15093), to work around a mypy caching bug:
https://github.com/python/mypy/issues/7281.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

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