PyMuPDF 1.19.4

Minor bug fixes and enhancements

on Python PyPI

Fixes: #1505, #1484, #1479, #1474.

Changes:

• Full support of PDF page rectangles like /ArtBox etc.
• New global variable TESSDATA_PREFIX for comfortably checking presence of OCR support
• Changed Document.xref_set_key() such that dictionary keys will physically be removed if set to value "null".
• Changed Document.extract_font() to optionally return a dictionary (instead of a tuple).