use TextIOWrapper when communicating with pandoc subprocess

uses 'universal-newline' mode, to avoid confusion with CRLF.

closes  #4122

IPython/nbconvert/utils/pandoc.py

@@ -1,69 +1,70 b''
 """Utility for calling pandoc"""
 #-----------------------------------------------------------------------------
 # Copyright (c) 2013 the IPython Development Team.
 #
 # Distributed under the terms of the Modified BSD License.
 #
 # The full license is in the file COPYING.txt, distributed with this software.
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 
 from __future__ import print_function
 
 # Stdlib imports
 import subprocess
+from io import TextIOWrapper, BytesIO
 
 # IPython imports
 from IPython.utils.py3compat import cast_bytes
 
 from .exceptions import ConversionException
 
 #-----------------------------------------------------------------------------
 # Classes and functions
 #-----------------------------------------------------------------------------
 
 class PandocMissing(ConversionException):
     """Exception raised when Pandoc is missing. """
     pass
 
 
 def pandoc(source, fmt, to, extra_args=None, encoding='utf-8'):
     """Convert an input string in format `from` to format `to` via pandoc.
 
     This function will raise an error if pandoc is not installed.
     Any error messages generated by pandoc are printed to stderr.
 
     Parameters
     ----------
     source : string
       Input string, assumed to be valid format `from`.
     fmt : string
       The name of the input format (markdown, etc.)
     to : string
       The name of the output format (html, etc.)
 
     Returns
     -------
     out : unicode
       Output as returned by pandoc.
     """
     command = ['pandoc', '-f', fmt, '-t', to]
     if extra_args:
         command.extend(extra_args)
     try:
         p = subprocess.Popen(command,
                              stdin=subprocess.PIPE, stdout=subprocess.PIPE
         )
     except OSError as e:
         raise PandocMissing(
             "The command '%s' returned an error: %s.\n" %(" ".join(command), e) +
             "Please check that pandoc is installed:\n" +
             "http://johnmacfarlane.net/pandoc/installing.html"
         )
     out, _ = p.communicate(cast_bytes(source, encoding))
-    out = out.decode(encoding, 'replace')
-    return out[:-1]
+    out = TextIOWrapper(BytesIO(out), encoding, 'replace').read()
+    return out.rstrip('\n')