easy fixes for PyPDF2 3.0.0 support

[cjfp]

On my system pdftools is installed at:

/usr/local/lib/python3.8/site-packages/pdftools/pdftools.py

If I edit this file as follows:

s/PdfFileReader/PdfReader/g
s/PdfFileWriter/PdfWriter/g
s/addPage/add_page/g

then the deprecation errors I get from PyPDF2 3.0.0 go away and things work as expected when merging PDFs (so far). There might be other updates required, of course, but hopefully find and replace is sufficient.

Note: there is an open pull request that also includes these fixes, as well as a move from PyPDF2 to pypdf. So the point of this issue is just to document a quick local workaround.