Installation is failing if jupyter notebook isn't already installed

[rlaverde]

I experimented this error when runing pip install .

This is caused because __init__ file is executed when importing __version__ in setup.py

Processing /home/rlaverde/code/spyder-notebook
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-_jv2fvz2-build/setup.py", line 17, in <module>
        from spyder_notebook import __version__
      File "/tmp/pip-_jv2fvz2-build/spyder_notebook/__init__.py", line 11, in <module>
        from spyder_notebook.notebookplugin import NotebookPlugin as PLUGIN_CLASS
      File "/tmp/pip-_jv2fvz2-build/spyder_notebook/notebookplugin.py", line 31, in <module>
        from .utils.nbopen import nbopen, NBServerError
      File "/tmp/pip-_jv2fvz2-build/spyder_notebook/utils/nbopen.py", line 16, in <module>
        from notebook import notebookapp
    ImportError: No module named 'notebook'

[ccordoba12]

Nice catch @rlaverde! Please send us a PR to fix this.

My proposal: Move this plugin version out of __init__ to a _version.py file.