follow PEP 639 for licenses (#541)

The license table with `file` key is now deprecated in favor of an SPDX
identifier and separate field for license-files.

https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license-and-license-files

Signed-off-by: Spencer Schrock <[email protected]>

Author: spencerschrock

pyproject.toml

@@ -1,13 +1,14 @@
 [build-system]
-requires = ["hatchling>=1.25"]
+requires = ["hatchling>=1.27"]
 build-backend = "hatchling.build"
 
 [project]
 name = "model-signing"
 dynamic = ["version"]
 description = "A tool for signing and verifying ML models"
 readme = "README.md"
-license = { file = "LICENSE" }
+license = "Apache-2.0"
+license-files = ["LICENSE"]
 authors = [
   { name = "Sigstore Authors", email = "[email protected]" }
 ]