Add SPDX license identifier (#401)

Author: cdce8p

pyproject.toml

@@ -1,13 +1,13 @@
 [project]
 name = "python-engineio"
 version = "4.12.1.dev0"
+license = {text = "MIT"}
 authors = [{name = "Miguel Grinberg", email = "miguel.grinberg@gmail.com"}]
 description = "Engine.IO server and client for Python"
 classifiers = [
     "Environment :: Web Environment",
     "Intended Audience :: Developers",
     "Programming Language :: Python :: 3",
-    "License :: OSI Approved :: MIT License",
     "Operating System :: OS Independent",
 ]
 requires-python = ">=3.6"