Replace pypdf with pyHanko

[bgermann]

Motivation

Both pypdf and pyHanko are dependencies. pyHanko includes a fork of PyPDF2, which is itself a fork of pypdf.

Proposed Solution

The pyHanko variants of the used pypdf objects (PdfReader, PdfWriter, PageObject) should be used.

Additional Context

Implementing this proposal enables to drop the pypdf dependency.

[stefan6419846]

PyPDF2, which is itself a fork of pypdf

This is wrong for some time now: PyPDF2 has been deprecated in favor of pypdf. pyHanko does indeed include a heavily modified version of PyPDF2.

In my personal opinion, 6777fe9 should be mostly reverted and pyHanko dropped completely for a clean separation of concerns.

[bgermann]

It would also be useful if pyHanko was optional instead of removing the whole feature.