Fix pyproject license (#324)

These parse out into core metadata `License` and `License-Expression`
fields.

Ref:
https://packaging.python.org/en/latest/specifications/pyproject-toml/#license

Author: stefanvanburen

pyproject.toml

@@ -6,8 +6,8 @@ build-backend = "hatchling.build"
 name = "protovalidate"
 description = "Protocol Buffer Validation for Python"
 readme = "README.md"
-license-expression = "Apache 2.0"
-license-file = "LICENSE"
+license = "Apache 2.0"
+license-files = ["LICENSE"]
 keywords = ["validate", "protobuf", "protocol buffer"]
 requires-python = ">=3.9"
 classifiers = [