downloads: upgrade to CPython 3.9.0 final

This was released today. Let's use the final release instead of the
release candidate.

Author: indygreg

pythonbuild/downloads.py

@@ -71,10 +71,10 @@
         "python_tag": "cp38",
     },
     "cpython-3.9": {
-        "url": "https://www.python.org/ftp/python/3.9.0/Python-3.9.0rc2.tar.xz",
-        "size": 18802576,
-        "sha256": "80b57c11f60dc1f46a408b1543f04ed52e6475ed5e597b4c23f3fd65f0b729ba",
-        "version": "3.9.0rc2",
+        "url": "https://www.python.org/ftp/python/3.9.0/Python-3.9.0.tar.xz",
+        "size": 18866140,
+        "sha256": "9c73e63c99855709b9be0b3cc9e5b072cb60f37311e8c4e50f15576a0bf82854",
+        "version": "3.9.0",
         "licenses": ["Python-2.0", "CNRI-Python"],
         "license_file": "LICENSE.cpython.txt",
         "python_tag": "cp39",