Resolve "Parts of the pyproject.toml are deprecated"

Author: btschwertfeger

pyproject.toml

@@ -20,7 +20,7 @@ maintainers = [
 ]
 description = "A collection of bias correction techniques written in Python - for climate sciences."
 readme = "README.md"
-license = { file = "LICENSE" }
+license-files = ["LICENSE"]
 requires-python = ">=3.9"
 dependencies = [
   "xarray>=2022.11.0",
@@ -44,7 +44,6 @@ keywords = [
   "quantile-delta-mapping",
 ]
 classifiers = [
-  "License :: OSI Approved :: GNU General Public License v3 (GPLv3)",
   "Programming Language :: Python",
   "Programming Language :: Python :: 3.9",
   "Programming Language :: Python :: 3.10",