Modernize license metadata

PEP-639 license metadata is now supported by hatchling, twine, pypi
release action and pip.
 * license field is an SPDX license expression
 * license-files lists all the license files included in distribution

Signed-off-by: Jussi Kukkonen <[email protected]>

Author: jku

pyproject.toml

@@ -8,7 +8,8 @@ build-backend = "hatchling.build"
 [project]
 name = "securesystemslib"
 authors = [{name = "https://www.updateframework.com", email = "[email protected]"}]
-license = {text = "MIT"}
+license = "MIT"
+license-files = [ "LICENSE" ]
 description = "A library that provides cryptographic and general-purpose routines for Secure Systems Lab projects at NYU"
 readme = "README.md"
 keywords = [
@@ -22,7 +23,6 @@ keywords = [
 classifiers = [
     "Development Status :: 4 - Beta",
     "Intended Audience :: Developers",
-    "License :: OSI Approved :: MIT License",
     "Natural Language :: English",
     "Operating System :: POSIX",
     "Operating System :: POSIX :: Linux",