Bibtextools

Added
- Add `KEY_EDITOR` and `KEY_BOOKTITLE` keywords to constants
- Add `clean_bib_file.get_duplicate_entries` function that returns duplicate
entries based on their similar titles, authors, and other information
- Add `clean_bib_file.remove_duplicate_entries` function that removes duplicate
entries based on the content (similar titles, authors, ...). Additionally,
this function is applied to all main functions after loading the bib files.
- Add new `--force` option for the CLI commands, which skip the interactive
prompt for removing duplicate citations.

Changed
- Rename the `clean_bib_file.get_duplicates` function to
`clean_bib_file.get_duplicate_ids`.
- Rename the `clean_bib_file.replace_duplicates` function to
`clean_bib_file.replace_duplicate_ids`.
- Move `modernize_bib_file.getnames()` to `util.getnames()`

Added
- Added function to abbreviate journal names based on the ISO4 standard using
the `pyiso4` implementation as part of the `modernize` command.

Changed
- Update packaging structure and metadata from old `setup.py` to
`pyproject.toml`
- Change minimum Python version requirement to 3.8 due to the pyiso4 dependency

Added
- A completely refactored command line interface using the `click` library is
added. This will replace the current CLI in a future version.

Fixed
- Fixed a bug in getting author names when shielded umlauts were already
present.

Added
- It is now possible to change the sorting of the entries when writing to a
file
- Added the `shield_title` argument for modernizing the title field, which
allows adding/removing curly brackets around the title field.
The argument can be used by setting the `--shield_title` flag in the
`modernize` command.

Changed
- The fields `keyword` and `keywords` do not get removed by default
- Surrounding curly brackets in the title field are removed by default. Use the
`--shield_title` argument to avoid this.

Fixed
- Already converted unicode symbols are not touched when cleaning unicode
characters.
- Improve author name cleaning in `modernize` subcommand. Names that are
shielded by `{}` are now split into first and last names respecting the
grouping.

Fixed
- Acronyms that are already shielded by `{}` will not be shielded again when
modernizing a bib file.