Fix SPDX license identifier: Python-2.0 → PSF-2.0

The SPDX identifier "Python-2.0" refers to the Python 2.0 license,
which is not GPL-compatible.  The correct identifier for the PSF
license used by greenlet is "PSF-2.0", which is GPL-compatible.

Backport of 77e65f0 from master.

Made-with: Cursor

Author: nbouvrette

setup.py

@@ -225,7 +225,7 @@ def get_greenlet_version():
         'Documentation': 'https://greenlet.readthedocs.io/',
         'Changes': 'https://greenlet.readthedocs.io/en/latest/changes.html',
     },
-    license="MIT AND Python-2.0",
+    license="MIT AND PSF-2.0",
     license_files=[
         'LICENSE',
         'LICENSE.PSF',