Fix: fix setuptools deprecation warning (#589)

Use the SPDX expression for license
classifier and remove the now deprecated
TOML table license according to this warning
in the CI jobs:

        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 2027-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.

and:

dist.py:765: SetuptoolsDeprecationWarning: License classifiers are deprecated.
!!

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

        License :: OSI Approved :: MIT License

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

!!

Author: pjonsson

pyproject.toml

@@ -11,7 +11,7 @@ name = "GeoAlchemy2"
 dynamic = ["version"]
 description = "Using SQLAlchemy with Spatial Databases"
 readme = "README.rst"
-license = {text = "MIT"}
+license = "MIT"
 requires-python = ">=3.10"
 authors = [
     {name = "Eric Lemoine", email = "eric.lemoine@gmail.com"},
@@ -28,7 +28,6 @@ classifiers = [
     "Programming Language :: Python :: 3.12",
     "Programming Language :: Python :: 3.13",
     "Intended Audience :: Information Technology",
-    "License :: OSI Approved :: MIT License",
     "Topic :: Scientific/Engineering :: GIS",
 ]
 dependencies = [