Update license metadata field

Author: bodograumann

README.md

@@ -85,7 +85,7 @@ Publishing
 
 We currently only publish source distributions.
 ```sh
-pip install twine
+pip install setuptools twine
 python setup.py sdist
 twine upload dist/*
 ```

pyproject.toml

@@ -8,14 +8,13 @@ version = "1.1.4"
 description = "iconv-based Unicode converter"
 readme = "README.md"
 requires-python = ">=3.6"
-license = {text = "GPL-3.0-or-later"}
+license = "GPL-3.0-or-later"
 maintainers = [
     {name = "Bodo Graumann", email = "mail@bodograumann.de"}
 ]
 classifiers = [
     "Development Status :: 5 - Production/Stable",
     "Intended Audience :: Developers",
-    "License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)",
     "Operating System :: POSIX",
     "Programming Language :: C",
     "Programming Language :: Python :: 3",