Backport PR #2778: P3K: fix DeprecationWarning under Python 3.x

The warning is:

```
.../site-packages/IPython/utils/jsonutil.py:121: DeprecationWarning: encodestring() is a deprecated alias, use encodebytes()
  encoded['image/png'] = encodestring(pngdata).decode('ascii')
```

IPython/utils/jsonutil.py

@@ -15,9 +15,15 b' import math'
 import re
 import sys
 import types
-from base64 import encodestring
 from datetime import datetime
 
+try:
+    # base64.encodestring is deprecated in Python 3.x
+    from base64 import encodebytes
+except ImportError:
+    # Python 2.x
+    from base64 import encodestring as encodebytes
+
 from IPython.utils import py3compat
 from IPython.utils.encoding import DEFAULT_ENCODING
 from IPython.utils import text
@@ -118,10 +124,10 b' def encode_images(format_dict):'
     encoded = format_dict.copy()
     pngdata = format_dict.get('image/png')
     if isinstance(pngdata, bytes) and pngdata[:8] == PNG:
-        encoded['image/png'] = encodestring(pngdata).decode('ascii')
+        encoded['image/png'] = encodebytes(pngdata).decode('ascii')
     jpegdata = format_dict.get('image/jpeg')
     if isinstance(jpegdata, bytes) and jpegdata[:2] == JPEG:
-        encoded['image/jpeg'] = encodestring(jpegdata).decode('ascii')
+        encoded['image/jpeg'] = encodebytes(jpegdata).decode('ascii')
     return encoded