Pandas-indexing

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* **BREAKING**: Change :func:`~core.extractlevel` to drop split levels by default and
accordingly rename the governing argument from ``drop=False`` to ``keep=False``
:pull:`53`.
* Add ``regex=True`` argument to :func:`~core.extractlevel` to use templates as
manual extraction regex, f.ex.
``df.pix.extract(variable=r"Emissions\|(?P<gas>.*?)(?:\|(?P<sector>.*?))?",
regex=True)`` will also split ``Emissions|CO2`` to ``gas = "CO2"`` and
``sector = NaN``, while ``df.pix.extract(variable="Emissions|{gas}|{sector}")`` would
have dropped it.
* Update :func:`~core.projectlevel` to raise ``KeyError`` for wrong level names
:pull:`52`.

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* Add :func:`~core.add_zeros_like` for adding explicit ``levels`` as 0 values :pull:`51`

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* Add :func:`~core.antijoin` for performing anti-joins :pull:`48`
* Update usage guide for ``antijoin``, but also with more focus on ``extractlevel``

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* **BREAKING** :mod:`accessors` is imported implicitly. User code does not need to
import it any longer :pull:`45`
* Add continuous testing for python 3.12 :pull:`47`
* Add :attr:`~selectors.All` and :attr:`~selectors.None_` completing selector group
:pull:`46`
* Fix type hints of function :func:`~core.aggregatelevel` :pull:`44`
* Switch from black to ruff for formatting and update pre-commit versions :pull:`43`

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* The new :func:`~core.assignlevel` argument ``ignore_index=True`` prevents the
dataframe and series alignment which became the default in v0.3 (yesterday),
since there are valid use cases of the old behaviour :pull:`41`

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* **BREAKING** :func:`~core.assignlevel` aligns :class:`~pandas.Series` and
:class:`~pandas.DataFrame` before adding them to the index :pull:`38`
* Address pandas 2.1's ``FutureWarning``s and improve test coverage :pull:`40`