Update project license configuration in `pyproject`

See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license

Warning presented in tox -e check:

/private/var/folders/5j/xnd974x90nv26p17z3b28rdr0000gn/T/build-env-cjl4bc7m/lib/python3.11/site-packages/setuptools/config/_apply_pyprojecttoml.py:82: SetuptoolsDeprecationWarning: `project.license` as a TOML table is deprecated
!!

        ********************************************************************************
        Please use a simple string containing a SPDX expression for `project.license`. You can also use `project.license-files`. (Both options available on setuptools>=77.0.0).

        By 2026-Feb-18, you need to update your project and remove deprecated calls
        or your builds will no longer be supported.

        See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
        ********************************************************************************

!!
  corresp(dist, value, root_dir)
/private/var/folders/5j/xnd974x90nv26p17z3b28rdr0000gn/T/build-env-cjl4bc7m/lib/python3.11/site-packages/setuptools/config/_apply_pyprojecttoml.py:61: SetuptoolsDeprecationWarning: License classifiers are deprecated.
!!

        ********************************************************************************
        Please consider removing the following classifiers in favor of a SPDX license expression:

        License :: MIT

        See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
        ********************************************************************************