[ENH] Move to pyproject.toml

[Moritz-Alexander-Kern]

This PR moves Elephant from setup.py to pyproject.toml, aligning with PEP 518.

Key changes include:

• Introduction of pyproject.toml
• setup.py is still needed to compile the C-code for the fim module.

The reason is that we need OS dependend compile args which are dynamically defined in setup.py. The pyproject.toml does not natively support conditional logic like os_name == 'windows' directly within the configuration. The TOML format is static and doesn't allow for dynamic evaluation of conditions based on the operating system.

To achieve platform-specific compile arguments, we have to use a Python script, such as a setup.py, which can programmatically determine the OS and apply the correct compile arguments.

In this context, as a configuration file for setuptools, setup.py is not deprecated, see also: https://packaging.python.org/en/latest/discussions/setup-py-deprecated/