Backport PR #10558: Let nbformat take care of opening the file

Notebook files should always be read as utf-8, but the default for `io.open()` is to use a platform-dependent default encoding. The easiest way around this is to pass the filename to nbformat.read() and let it open the file correctly.

Bug identified at: http://stackoverflow.com/questions/43915006/encoding-error-in-jupyter-when-run-another-notebook

IPython/core/interactiveshell.py

@@ -2535,8 +2535,7 class InteractiveShell(SingletonConfigurable):
             """generator for sequence of code blocks to run"""
             if fname.endswith('.ipynb'):
                 from nbformat import read
-                with io_open(fname) as f:
-                    nb = read(f, as_version=4)
+                nb = read(fname, as_version=4)
                     if not nb.cells:
                         return
                     for cell in nb.cells: