examples/tests/whiteboard.py: use bytes on base64.encodestring()

base64.encodestring() requires bytes on Python 3, and the whiteboard,
on the other hand, requires a string.

The default encodings will be used for encoding, and for encoding,
we want to be safe and use ascii which can be represented in the
various result formats.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

examples/tests/whiteboard.py

@@ -38,7 +38,7 @@ class WhiteBoard(Test):
         result = ''
         for _ in range(0, iterations):
             result += data
-        self.whiteboard = base64.encodestring(result)
+        self.whiteboard = base64.encodestring(result.encode()).decode('ascii')
 
 
 if __name__ == "__main__":