Remove references to _Sentinel in type hints of public methods
Remove references to _Sentinel in type hints of public methods

Change the original methods (with `_T` typing) from public to protected.

Add new public methods (with `T` typing) that delegate to the protected
ones.

Refactor to internally use the protected methods, in order to flatten
the call stack or avoid repeated/unnecessary assertions.
madman-bob/python-order-maintenance