tests fail with pypdf2 1.x versions

I suspect 6aed2e53 broke support for older pypdf2 versions, as when I upgraded to diffoscope 233 in guix, it caused test suite failures. The fix for guix was to switch to pypdf 3.x (not to be confused with pypdf2 3.x), as guix had an older pypdf2 1.x version, so in a sense the issue is settled at least for guix.

That said, seems like it would be good to either make the tests backwards compatible with older pypdf2 again (older pypdf2 1.x worked with diffoscope 224 fine), or mark the tests as requiring a newer version.

I will dig up more details, just wanted to get this reported before I forget...

p.s. If you like being confused by versioning/naming discrepancies, check out https://pypi.org/search/?q=pypdf ... fork early, fork often, fork confusingly