Python dependency declaration

[atreyasha]

Thanks for this tool, very helpful for Bitwarden migration!

Thinking it could make sense to add a pyproject.toml for this project, so new users can install a virtual environment using eg. poetry, and then use eg. poetry run ./pass2bw.py. Ths is advantageous as you don't mess with your system's python environment.

WDYT?

PS: This is an initial diff, would still need to update package authors and readme.