DeprecationWarning from traitlets

[NathanDunfield]

With DeprecationWarnings on, I get the following warning message with ipympl 0.1.1 and traitlets 4.3.2:

/sage/local/lib/python2.7/site-packages/ipympl/backend_nbagg.py:125: 
DeprecationWarning: metadata {'sync': True} was set from the constructor. 
With traitlets 4.1, metadata should be set using the .tag() method, e.g., 
Int().tag(key1='value1', key2='value2')
  _model_module = Unicode('jupyter-matplotlib', sync=True)

So line 125 (and the subsequent similar code) should be updated to

  _model_module = Unicode('jupyter-matplotlib').tag(sync=True)

The current ipympl requires ipywidgets>=7.0 which in turn requires traitlets>=4.3.1, so there is no need to support traitlets < 4.1.

[SylvainCorlay]

Thanks @NathanDunfield! Good catch.

Feel free to open a PR. I would be happy to merge.

[NathanDunfield]

@SylvainCorlay PR created!

[SylvainCorlay]

Thanks!