Use pyproject.toml to install pybind11

There is a catch 22 atm in the setup.py file, as pybind11 is required to be able to run setup.py (and install pybind), as discussed here. There are two main solutions to this problem, outlined below. This MR implements solution 1), which is recommended for modern projects.

Copied from the other discussion:

There seems to be two main encouraged ways of solving this problem. 1) Use pyproject.toml to make sure that pybind11 is available. This is the new recommended way of doing things, but it requires pip 10+. 2) Use setup_requires, which will work for old pip versions, but is more of a hack. In calorine we support Python >= 3.8 which is from 2019, pip 10 is from 2018 afaict. So I would suggest us going with option 1).

Closes #67 (closed)