Merge PyPDF4 changes back to PyPDF2

[MartinThoma]

As PyPDF2 is maintained again (I'm the new maintainer) and it seems like PyPDF4 still is way smaller, I'd be happy if the two communities merged together again. I'm still struggling with lots of PRs on PyPDF2 side, but please feel free to add small ones there again :-)

A good PR has the following properties:

1. It does one thing, and does it well: Try to keep the size of the PR small
2. It explains a little bit what it's about. Is it a bugfix? Is it a feature? Just a stylistic change? Why is it a good idea to add it?
3. It contains a unit test. If it's a bugfix, it contains a test that now succeeds but was failing before.