convert: change socket mode from b'r+' to 'rwb' in cvs.py (issue6789)

'r+' mode used to open sockets for read/write operations, but '+' is not
supported in Python 3. We're using bytes with these sockets everywhere, so the
mode should have 'b'.  But the mode argument has to be str, not bytes.

hgext/convert/cvs.py

@@ -199,7 +199,7 b' class convert_cvs(converter_source):'
                 if sck.recv(128) != b"I LOVE YOU\n":
                     raise error.Abort(_(b"CVS pserver authentication failed"))
 
-                self.writep = self.readp = sck.makefile(b'r+')
+                self.writep = self.readp = sck.makefile('rwb')
 
         if not conntype and root.startswith(b":local:"):
             conntype = b"local"