fix: add Python version classifiers to pyproject.toml

The shields.io pypi/pyversions badge reads 'Programming Language :: Python'
classifiers from PyPI. Without them it displays 'python: missing'.
Adds classifiers for Python 3.10–3.13 matching requires-python.

Author: Brad Kinnard

pyproject.toml

@@ -17,6 +17,18 @@ dependencies = [
     "pyyaml>=6.0",
 ]
 
+classifiers = [
+    "Development Status :: 5 - Production/Stable",
+    "Intended Audience :: Developers",
+    "License :: OSI Approved :: MIT License",
+    "Programming Language :: Python :: 3",
+    "Programming Language :: Python :: 3.10",
+    "Programming Language :: Python :: 3.11",
+    "Programming Language :: Python :: 3.12",
+    "Programming Language :: Python :: 3.13",
+    "Topic :: Software Development :: Quality Assurance",
+]
+
 [project.optional-dependencies]
 dev = [
     "mypy>=1.10",