Migrate setup.py to pyproject.toml

[crosenth]

pip --editable (setup.py devel) is deprecated and we should move from setup.py to pyproject.toml anyway.

Some additional notes:

pypa/pip#11457

% cd /src/taxtastic/
% pip install --editable .
DEPRECATION: Legacy editable install of taxtastic==0.10.1+52.g0d37ae3.dirty from file:///mnt/home/crosenth/src/taxtastic (setup.py develop) is deprecated. pip 25.1 will enforce this behaviour change. A possible replacement is to add a pyproject.toml or enable --use-pep517, and use setuptools >= 64. If the resulting installation is not behaving as expected, try using --config-se
ttings editable_mode=compat. Please consult the setuptools documentation for more information. Discussion can be found at https://github.com/pypa/pip/issues/11457