textproc/py-ocrmypdf: Update to 16.3.1

• Add a workaround to fallback to graphics/py-pillow-heif if the pi_heif module isn't available to enable support for converting HEIF/HEIC images.

• Remove no longer needed requirements/workarounds while I'm here, as OCRmyPDF requires at least Python 3.10 or newer since version 16.x.

Changelog since 15.4.4:

https://github.com/ocrmypdf/OCRmyPDF/blob/v16.3.1/docs/release_notes.rst

PR: 276310
Reported by: wen
Co-Authored by: wen (initial patch for 16.0.4)
MFH: No (2024Q4 has Python 3.9 as default)