Safer loading of default config inside Python 3 config.

IPython/config/profile/python3/ipython_config.py

@@ -1,3 +1,11 b''
-# This can be used at any point in a config file to load a sub config
-# and merge it into the current one.
-load_subconfig('ipython_config.py', profile='default')
+c = get_config()
+
+# If the master config file uses syntax that's invalid in Python 3, we'll skip
+# it and just use the factory defaults.
+try:
+    load_subconfig('ipython_config.py', profile='default')
+except Exception:
+    pass
+else:
+    # We reset exec_lines in case they're not compatible with Python 3.
+    c.InteractiveShellApp.exec_lines = []