Convert the socket module to insist on bytes for input, and to return bytes

(not bytearray) on output.  Discovered a bunch of places that were still
depending on it accepting text strings.

Lib/test/test_poplib.py

@@ -19,7 +19,7 @@ def server(ready, evt):
     except socket.timeout:
         pass
     else:
-        conn.send("+ Hola mundo\n")
+        conn.send(b"+ Hola mundo\n")
         conn.close()
     finally:
         serv.close()