Add missing import of py3compat

IPython/core/application.py

@@ -38,6 +38,7 b' from IPython.config.application import Application, catch_config_error'
 from IPython.config.loader import ConfigFileNotFound
 from IPython.core import release, crashhandler
 from IPython.core.profiledir import ProfileDir, ProfileDirError
+from IPython.utils import py3compat
 from IPython.utils.path import get_ipython_dir, get_ipython_package_dir
 from IPython.utils.traitlets import List, Unicode, Type, Bool, Dict, Set, Instance