publish deprecated Sentinel

this was accidentially published before, we can't un-publish it now since that's a breakage
but publish with a deprecation warning

traitlets/__init__.py

@@ -1,3 +1,19 @@
+from warnings import warn
+
+from . import traitlets
 from .traitlets import *
 from .utils.importstring import import_item
 from ._version import version_info, __version__
+
+
+class Sentinel(traitlets.Sentinel):
+    def __init__(self, *args, **kwargs):
+        super(Sentinel, self).__init__(*args, **kwargs)
+        warn(
+            """
+            Sentinel is not a public part of the traitlets API.
+            It was published by mistake, and may be removed in the future.
+            """,
+            DeprecationWarning,
+            stacklevel=2,
+        )