Log Message:
py-pybtex: updated to 0.24.0

Version 0.24.0
--------------
**This is the last version that supports Python 2. The next version will
require Python 3.6 or above.**

- Added support for ``sup`` and ``sub`` tags to LaTeX and Markdown backends.
- Added support for ``@online`` entries and the ``urldate`` field.
- Restored compatibility with Python 2.
- Fixed tests on Windows.
- Fixed bugs in the example plugin.
- Fixed bad ``get_default_encoding()`` call.