add support for new Python pyproject.toml standard: setup.py install is deprecated

[pabs3]

When I build the Debian package of git-imerge with Python 3.11 and setuptools 67.8.0 I get a warning about setup.py install being deprecated. Please consider adding support for pyproject.toml (setuptools pyproject.toml guide) so that users and distros can build/install git-imerge without using the setup.py script.

/usr/bin/python3 setup.py install --root /home/pabs/devel/debian/misc/git-imerge/git-imerge-1.2.0/debian/git-imerge 
running install
/usr/lib/python3/dist-packages/setuptools/_distutils/cmd.py:66: SetuptoolsDeprecationWarning: setup.py install is deprecated.
!!

        ********************************************************************************
        Please avoid running ``setup.py`` directly.
        Instead, use pypa/build, pypa/installer, pypa/build or
        other standards-based tools.

        See https://blog.ganssle.io/articles/2021/10/setup-py-deprecated.html for details.
        ********************************************************************************

!!
  self.initialize_options()