Update pyproject license schema according to deprecation warnings

* Building wheel...
/tmp/build-env-vi5sasbz/lib/python3.12/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)
/tmp/build-env-vi5sasbz/lib/python3.12/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 :: OSI Approved :: BSD License
        See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
        ********************************************************************************

Author: maresb

pyproject.toml

@@ -14,13 +14,13 @@ requires-python = ">=3.10,<3.14"
 authors = [{ name = "pymc-devs", email = "pymc.devs@gmail.com" }]
 description = "Optimizing compiler for evaluating mathematical expressions on CPUs and GPUs."
 readme = "README.rst"
-license = { file = "LICENSE.txt" }
+license = "BSD-3-Clause"
+license-files = ["LICENSE.txt"]
 classifiers = [
     "Development Status :: 6 - Mature",
     "Intended Audience :: Education",
     "Intended Audience :: Science/Research",
     "Intended Audience :: Developers",
-    "License :: OSI Approved :: BSD License",
     "Programming Language :: Python",
     "Topic :: Software Development :: Code Generators",
     "Topic :: Software Development :: Compilers",