Introduce pyproject.toml

[BenjaminRodenberg]

It is recommended to use pyproject.toml.

I would suggest to start by managing the sollowing additional points over pyproject.toml:

• Dependencies
  • mpi4py < 4 is required. See Add missing requirements.txt and venvs for Python tutorials tutorials#563
  • Add fenics as a dependency? Note that the installation process of FEniCS is somewhat non-standard. See, for example, what I'm doing here
• According to https://ianhopkinson.org.uk/2022/02/understanding-setup-py-setup-cfg-and-pyproject-toml-in-python/ we can also replace the setup.cfg file with pyproject.toml. I introduced setup.cfg to be able to use versioneer. We should be able to move that to pyproject.toml (see https://github.com/python-versioneer/python-versioneer?tab=readme-ov-file#vendored-mode).
• Remove the python version checks from setup.py and into pyproject.toml. See https://stackoverflow.com/questions/76161889/how-do-i-specify-a-python-version-constraint-for-a-dependency-specifically-for-a

[fsimonis]

https://hatch.pypa.io/latest/intro/#existing-project
This can convert the setup.py to pyproject.toml