py-pybtex: updated to 0.23.0

Version 0.23.0
--------------

- Reimplemented :py:class:`~pybtex.utils.OrderedCaseInsensitiveDict` using
  :py:class:`python:collections.OrderedDict` (so it has a __delitem__).

- ``unsrt.py`` now supports type when formatting phdthesis.

- Added :py:meth:`~pybtex.database.BibliographyData.from_string` to
  :py:class:`pybtex.database.BibliographyData`.

- Added :py:meth:`~pybtex.database.Entry.from_string` and
  :py:meth:`~pybtex.database.Entry.to_string` to
  :py:class:`pybtex.database.Entry`.

- Added indentation to ``__repr__`` in
  :py:class:`pybtex.database.BibliographyData` and
  :py:class:`pybtex.database.Entry`.

- Preserve order in
  :py:meth:`pybtex.utils.OrderedCaseInsensitiveDict.__repr__`.

- Fixed entries with duplicate keys being removed during sorting.

- Fixed handling of duplicate person fields

- Use :py:mod:`~xml.etree.ElementTree` instead of the deprecated
  ``cElementTree``.

- Import base classes from :py:mod:`python:collections.abc` instead of
  :py:mod:`python:collections`.

- Use ``__iter__`` instead of deprecated ``Element.getchildren()``.

(adam)