python 3.10 officially supported

we test on py310-dev on github CI since a while and now also on the vagrant machines, so it should work ok.

setup.py

@@ -882,6 +882,7 @@ setup(
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
         'Programming Language :: Python :: 3.9',
+        'Programming Language :: Python :: 3.10',
         'Topic :: Security :: Cryptography',
         'Topic :: System :: Archiving :: Backup',
     ],