pyproject: use SPDX license expression, remove classifier

This is apparently how you are supposed to do things now.

Author: indygreg

pyproject.toml

@@ -6,12 +6,11 @@ readme = { file = "README.rst", content-type = "text/x-rst" }
 authors = [
     { name = "Gregory Szorc", email = "gregory.szorc@gmail.com" },
 ]
-license = { text = "BSD" }
+license ="BSD-3-Clause"
 requires-python = ">=3.9"
 classifiers = [
     "Development Status :: 5 - Production/Stable",
     "Intended Audience :: Developers",
-    "License :: OSI Approved :: BSD License",
     "Programming Language :: C",
     "Programming Language :: Python :: 3.9",
     "Programming Language :: Python :: 3.10",