Add SPDX license identifier (#1453)

pyproject.toml

@@ -1,6 +1,7 @@
 [project]
 name = "python-socketio"
 version = "5.13.1.dev0"
+license = {text = "MIT"}
 authors = [
     { name = "Miguel Grinberg", email = "[email protected]" },
 ]
@@ -9,7 +10,6 @@ classifiers = [
     "Environment :: Web Environment",
     "Intended Audience :: Developers",
     "Programming Language :: Python :: 3",
-    "License :: OSI Approved :: MIT License",
     "Operating System :: OS Independent",
 ]
 requires-python = ">=3.8"