textproc/py-ocrmypdf: Update to 9.0.1

• Make the dependency for graphics/pngquant optional as pngquant is not strictly required but should be provided where possible.
• Set the new option as non-default for a while because there's a bug that leads to incorrect inverted PNG images in conjunction with pngquant.
• Install shell auto-completions to improve user experience.
• Remove the "slow" tests from the "do-test" target. There's very little point to run those at downstream.

Changelog since 8.3.2:

https://github.com/jbarlow83/OCRmyPDF/blob/v9.0.1/docs/release_notes.rst