Proxy issue for upstream issues in peekpdf & libqpdf

[huettenhain]

We currently require peekpdf<=9.5 in the xtpdf unit because of the following issues:

• https://github.com/pikepdf/pikepdf/issues/674
• https://github.com/qpdf/qpdf/issues/1552

We have multiple ways to resolve this issue, depending on the outcome. If the current behavior is intended, we can either keep the version requirement or re-introduce a dependency on pypdf2 and fall back to that one on error.