update code to use new pypdf2 API

[emmanuelrosa]

This commit updates the pypdf2 code to the new API.

This should take care of #35 and NixOS/nixpkgs#226012

NOTE: PDF optimizing is not working, but that's a different issue.

[arminstraub]

Thank you, Emmanuel! Things seem to have moved a lot in PyPDF2 land since I last had a chance to look at things. (My apologies for the long absence due to too little time.) As of now, the new version of PyPDF2 has been merged back into pypdf.

I have just made the required naming changes as in
https://pypdf.readthedocs.io/en/latest/user/migration-1-to-2.html
similar to what you did in this pull request. However, unlike your proposed code, I did not want to drop support for PyPDF2 in the process because that is currently the only version in the main repository of Ubuntu 20.04. The current version supports both.

Your help is much appreciated! I have added thanks to you in the ChangeLog :)

[jluttine]

@arminstraub Could you make a new release with this fix? The latest release v0.6.0 is more than three years old..