Ocrmypdf-papermerge

- Fix Image.ANTIALIAS deprication issue

Changed

- minor adjustment for new plugin interface i.e. incl.
``thresholding=options.tesseract_thresholding`` argument in ``tesseract.generate_hocr``

Changed

- readme field added to pyproject.toml file

Added

- changelog.md file

Changed

- use pytest instead of unittest
- use poetry for dependencies management
- updated dependencies