Eliminate non python dependencies

[milan-tom]

Summary

This uses the pypdfium2 module to eliminate the non-python module dependencies (pdftk and poppler_utils) in the pdfdeanimate-image.py script and improve performance.

Benefits

• Makes pdfdeanimate-image.py script much easier to set up and use for those who don't use nix-shell (can now install dependencies using pip install -r requirements.txt).
• Performance of the script significantly improved.
• No longer any need for creating an intermediate folder to store PDF page images.

Problems

• pypdfium2 has not yet been packaged for nix-shell, so this would have to be installed manually by nix-shell users.

[schokotets]

i merged this. thanks for your contribution!