gh-97590: Update docs and tests for ftplib.FTP.voidcmd() (GH-96825)

Since 2f3941d this function returns the
response string, rather than nothing.

Author: matthewhughes934

Doc/library/ftplib.rst

@@ -232,8 +232,8 @@ FTP objects
    .. method:: FTP.voidcmd(cmd)
 
       Send a simple command string to the server and handle the response.  Return
-      nothing if a response code corresponding to success (codes in the range
-      200--299) is received.  Raise :exc:`error_reply` otherwise.
+      the response string if the response code corresponds to success (codes in
+      the range 200--299).  Raise :exc:`error_reply` otherwise.
 
       .. audit-event:: ftplib.sendcmd self,cmd ftplib.FTP.voidcmd
 

Lib/test/test_ftplib.py

@@ -543,8 +543,8 @@ def test_set_pasv(self):
         self.assertFalse(self.client.passiveserver)
 
     def test_voidcmd(self):
-        self.client.voidcmd('echo 200')
-        self.client.voidcmd('echo 299')
+        self.assertEqual(self.client.voidcmd('echo 200'), '200')
+        self.assertEqual(self.client.voidcmd('echo 299'), '299')
         self.assertRaises(ftplib.error_reply, self.client.voidcmd, 'echo 199')
         self.assertRaises(ftplib.error_reply, self.client.voidcmd, 'echo 300')