Migrate to Poetry

[RobertRosca]

I was interested in this package and when trying to install via pipx noticed that none of the dependencies were installed due to #17 (fixed by #30).

Also the requirements weren't pinned and since this is a self-contained application, not a library meant to be imported, pinning all dependencies seemed like a good idea.

To do that I converted the project from setup.py to Poetry with a pyproject.toml file, committed the poetry.lock file so that the pinned packages are used, and changed the way the command line argument is exposed so that termpdf is added to bin during installation of the package.

Updated the readme to cover the new installation instructions.

Not sure if this would be accepted as it is a large change to how the project is packaged, so feel free to reject if you prefer setup.py.

[renzmann]

I think this is definitely preferable to what I did in #30. @dsanson it looks like at least a few people have had the same dependency problem based on this PR and the one I linked above. I think the migration proposed here would be worth it.