Update dropbox.py for compatibility with Dropbox SDK v11.X.X

`dropbox.dropbox` has been renamed to `dropbox.dropbox_client`

https://github.com/dropbox/dropbox-sdk-python/blob/main/UPGRADING.md

luigi/contrib/dropbox.py

@@ -30,7 +30,7 @@
 logger = logging.getLogger('luigi-interface')
 
 try:
-    import dropbox.dropbox
+    import dropbox.dropbox_client
     import dropbox.exceptions
     import dropbox.files
 except ImportError:
@@ -76,7 +76,7 @@ def __init__(self, token, user_agent="Luigi"):
             raise ValueError("The token parameter must contain a valid Dropbox Oauth2 Token")
 
         try:
-            conn = dropbox.dropbox.Dropbox(oauth2_access_token=token, user_agent=user_agent)
+            conn = dropbox.dropbox_client.Dropbox(oauth2_access_token=token, user_agent=user_agent)
         except Exception as e:
             raise Exception("Cannot connect to Dropbox. Check your Internet connection and the token. \n" + repr(e))