restoring BokehModel import in __init__.py (reverts part of 8f21a15)

Author: sdbbs

jupyter_bokeh/__init__.py

@@ -6,6 +6,7 @@
 
 HERE = Path(__file__).parent.resolve()
 
+from .widgets import BokehModel
 with (HERE / "labextension" / "package.json").open() as fid:
     data = json.load(fid)