I found most of deprecated items in my first comment are aliving by difficult reasons.

I grepped top 4000 packages and found some candidates to deprecate.


## turtle

* settiltangle is not used anywhere.
* tiltangle is also deprecated by docstring, but not in the document.
* Both methods don't emit DeprecationWarning.

TODO(easy): Update the document and emit DeprecationWarning


## email.errors

MalformedHeaderDefect is not used anywhere in top4000 packages.

But it is simple alias, and DeprecationWarning is not emit.

TODO(easy): Emit DeprecationWarning using module __getattr__.


## importlib

abc.SourceLoader.path_mtime is not used anywhere.

TODO: Remove path_mtime from the ABC and raise DeprecationWarning if path_mtime is defined in subclass in path_stats.

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importlib should be checked by experts.  I keep TODO(easy) for new contributors.