Make pip install work with new version of setuptools

[backmari]

Installing using pip install is failing with error messages like:

_MissingDynamic: license defined outside of pyproject.toml is ignored.

This seems related to a new release of setuptools: https://stackoverflow.com/a/77527178

This PR changes pyproject.toml to follow current standards to be able to do pip install.

To test, make sure that pip install works and that the package metadata is shown as expected, especially the dynamic metadata that is pulled from setup.cfg.

$ pip install -e .
...
$ pip show --verbose LiquidsReflectometer
Name: LiquidsReflectometer
Version: 2.0.13
Summary: Reduction scripts for the Liquids Reflectometer. This includes both automated reduction scripts and useful scripts to reprocess data.
Home-page: https://github.com/neutrons/LiquidsReflectometer
Author: Mat
Author-email: [email protected]
License: BSD 3-Clause License
Location: /home/u5z/mambaforge/envs/liqref/lib/python3.8/site-packages
Editable project location: /home/u5z/projects/LiquidsReflectometer
Requires: lmfit, mantidworkbench, matplotlib, numpy
Required-by: 
Metadata-Version: 2.1
Installer: pip
Classifiers:
  Operating System :: OS Independent
  Programming Language :: Python :: 3
Entry-points:
Project-URLs:

[jmborr]

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