Add "Python 3 Only" classifier

Author: Ickerday

pyproject.toml

@@ -17,6 +17,7 @@ authors = [{ name = "Splunk, Inc.", email = "[email protected]" }]
 keywords = ["splunk", "sdk"]
 classifiers = [
   "Programming Language :: Python :: 3",
+  "Programming Language :: Python :: 3 :: Only",
   "Programming Language :: Python :: 3.7",
   "Programming Language :: Python :: 3.9",
   "Programming Language :: Python :: 3.13",