[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/

[coveralls]

coverage: 88.257% (-0.1%) from 88.401%
when pulling 650edf2 on INM-6:enh/pyproject.toml
into 123ca04 on NeuralEnsemble:master.

[Moritz-Alexander-Kern]

Editable install with setup.py will be deprecated with pip 25.0

DEPRECATION: Legacy editable install of elephant==1.2.0b1 from file:/// (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 pypa/pip#11457