Add pyproject.toml file

[anomam]

Issue

When installing with pip install -e ".", I got

DEPRECATION: Legacy editable install of solarfactors==0+untagged.109.gc161def from file:///<...>/solarfactors (setup.py develop) is deprecated. pip 25.0 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-settings editable_mode=compat. Please consult the setuptools documentation for more information. Discussion can be found at https://github.com/pypa/pip/issues/11457

Suggested solution

Add a pyproject.toml file per recommended in the warning message.