update pyproject.toml to enable using poetry as dependency manager

[mjpan]

update pyproject.toml to enable using poetry as dependency manager. there's also a minor change to remove using the "background" parameter for the panel, as that seems to be deprecated

As alphaviz depends on alphatims, this pull request also depends on a related pull request in alphatims
MannLabs/alphatims#297

For a clean Python environment on Macs:

• Install Homebrew. Follow instructions on Homebrew
• Install mono using Homebrew. Follow instructions on mono
• Install poetry using Homebrew. Follow instructions on poetry

To run this using Poetry

• clone the alphatims repo that contains the pull request
• close this repo to sit next to alphatims
• in the cloned repo, check out the branch that contains the pull request
• install dependencies using poetry, run the tests, and run the gui
  poetry update
poetry run pytest
poetry run alphaviz-gui