ftplib.FTP.retrbinary callback gets a bytes, not a str (GH-652) (GH-657)
diff --git a/Doc/library/ftplib.rst b/Doc/library/ftplib.rst
index 1153ce7..7291dfe8 100644
--- a/Doc/library/ftplib.rst
+++ b/Doc/library/ftplib.rst
@@ -235,7 +235,7 @@
Retrieve a file in binary transfer mode. *cmd* should be an appropriate
``RETR`` command: ``'RETR filename'``. The *callback* function is called for
- each block of data received, with a single string argument giving the data
+ each block of data received, with a single bytes argument giving the data
block. The optional *blocksize* argument specifies the maximum chunk size to
read on the low-level socket object created to do the actual transfer (which
will also be the largest size of the data blocks passed to *callback*). A