Use SPDX license identifier (#16230)

It does not change the license itself, only its identifier in
`setup.py`, so external tools can read it better.

Full list: https://spdx.org/licenses/
Closes #16228

Author: sobolevn

setup.py

@@ -202,7 +202,7 @@ def run(self):
     author="Jukka Lehtosalo",
     author_email="[email protected]",
     url="https://www.mypy-lang.org/",
-    license="MIT License",
+    license="MIT",
     py_modules=[],
     ext_modules=ext_modules,
     packages=find_packages(),