gh-65772: Clean-up turtle module (#104218)

* Remove the unused, private, and undocumented name `_ver` and
the commented-out `print` call.

* Don't add math functions to `__all__`.  Beginners should learn
to `import math` to access them.

* Gregor Lindel, who wrote this version of turtle, dropped plans
to implement turtle on another toolkit at least a decade ago.
Drop `_dot` code preparing for this, but add a hint comment.

* `_Screen` is meant to be a singleton class.  To enforce that,
it needs either a `__new__` that returns the singleton or
`else...raise` in `__iter__`.  Merely removing the `if` clauses
as suggested might break something if a user were to call `_Screen`
directly.  Leave the code alone until a problem is evident.

* Turtledemo injects into _Screen both _root and _canvas,
configured as it needs them to be.  Making _canvas an `__init__`
option would require skipping some but not all of the lines under
'if _Screen._canvas is None:`.  Leave working code alone.