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Features
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- Add ``AbsenceFactory`` class for creating package-specific absence sentinels.
Each instance evaluates to ``False`` in boolean contexts, has unique identity,
and supports customizable string representations.
- Add ``Absential`` type alias for annotating values that may be absent,
supporting clear type hints for optional parameters and return values where
``None`` is a valid value.
- Add ``absent`` sentinel, a falsey singleton that represents absence in contexts
where ``None`` might be a valid value. The sentinel maintains global uniqueness,
supports proper equality comparison, and can be installed as a builtin.
- Add ``is_absent`` and ``is_absence`` predicates for type-safe checking of
absence sentinels. These can distinguish between the global sentinel and
factory-produced sentinels, and support installation as builtins.
- Add optional builtin installation support, allowing the ``absent`` sentinel
and its predicates to be used without imports, similar to ``None``. Supports
custom names and selective installation of components.
Supported Platforms
-------------------
- Add support for CPython 3.10 to 3.13.
- Add support for PyPy 3.10.