adjust default value of level in sync_imports

and value check for <= 0 instead of just -1.

Python 3 no longer allows negative values (relative or absolute),
and we should have always allowed 0 to work anyway.

closes  #4296

IPython/parallel/client/view.py

@@ -457,7 +457,7 b' class DirectView(View):'
             else:
                 user_ns[name] = sys.modules[name]
 
-        def view_import(name, globals={}, locals={}, fromlist=[], level=-1):
+        def view_import(name, globals={}, locals={}, fromlist=[], level=0):
             """the drop-in replacement for __import__, that optionally imports
             locally as well.
             """
@@ -478,7 +478,7 b' class DirectView(View):'
             imp.release_lock()
 
             key = name+':'+','.join(fromlist or [])
-            if level == -1 and key not in modules:
+            if level <= 0 and key not in modules:
                 modules.add(key)
                 if not quiet:
                     if fromlist: