os.genenvb() is not availabnle under Windows

Doc/library/os.rst

@@ -206,6 +206,9 @@ process and user.
    Return the value of the environment variable *key* if it exists, or
    *default* if it doesn't. *key*, *default* and the result are bytes.
 
+   :func:`getenvb` is only available if :data:`supports_bytes_environ`
+   is True.
+
    Availability: most flavors of Unix.
 
    .. versionadded:: 3.2