bpo-35950: Raise UnsupportedOperation in BufferedReader.truncate() (GH-18586)

The truncate() method of io.BufferedReader() should raise
UnsupportedOperation when it is called on a read-only
io.BufferedReader() instance.





https://bugs.python.org/issue35950



Automerge-Triggered-By: @methane

Lib/_pyio.py

@@ -792,6 +792,9 @@ class _BufferedIOMixin(BufferedIOBase):
         return pos
 
     def truncate(self, pos=None):
+        self._checkClosed()
+        self._checkWritable()
+
         # Flush the stream.  We're mixing buffered I/O with lower-level I/O,
         # and a flush may be necessary to synch both views of the current
         # file state.