add python3.10 tested note to docs

Author: wbarnha

docs/index.rst

@@ -11,7 +11,7 @@ Python bindings to the C++ interface of http://rocksdb.org/ using cython::
     db.put(b"a", b"b")
     print db.get(b"a")
 
-Tested with python3.8 and python3.9 and RocksDB version 6.11.4.
+Tested with python3.8, python3.9, python3.10, and RocksDB version 6.11.4.
 
 .. toctree::
     :maxdepth: 2