Add missing "import inspect"

[mgeier]

I got an error when running the master branch of notebook:

[W 15:12:59.786 NotebookApp] Error loading config file: jupyter_notebook_config
    Traceback (most recent call last):
...
      File "/home/[...]/notebook/notebookapp.py", line 1402, in _observe_contents_manager_class
        if inspect.isclass(new):
    NameError: name 'inspect' is not defined

I don't know why the import is missing how this hasn't been detected by CI, but adding the import statement seems to fix the error/warning.

[Zsailer]

Yikes, good catch, @mgeier. Thank you.