Use License Expressions in pyproject.toml

With the release of PEP-639 the best practice for specifying the license
is now to use a license expression in the license field and specify any
license files in license-files rather than the table-based approach from
PEP-621. Including the license in the classifiers is also no longer
allowed when using PEP-639 and has been removed.

Signed-off-by: Chris Marchbanks <csmarchbanks@gmail.com>

Author: csmarchbanks

pyproject.toml

@@ -7,7 +7,10 @@ name = "prometheus_client"
 version = "0.22.0"
 description = "Python client for the Prometheus monitoring system."
 readme = "README.md"
-license = { file = "LICENSE" }
+license = "Apache-2.0"
+license-files = [
+	"LICENSE",
+]
 requires-python = ">=3.9"
 authors = [
     { name = "The Prometheus Authors", email = "prometheus-developers@googlegroups.com" },
@@ -33,7 +36,6 @@ classifiers = [
     "Programming Language :: Python :: Implementation :: CPython",
     "Programming Language :: Python :: Implementation :: PyPy",
     "Topic :: System :: Monitoring",
-    "License :: OSI Approved :: Apache Software License",
 ]
 
 [project.optional-dependencies]