Fix param.ipython auto-import

[maximlt]

We noticed that import param wasn't enough to get the IPython display hook set up automatically while it should be given the code, instead the reactive user guide had to run import param.ipython for the hook to be set up.

There were two bugs, the first one being that error raised when trying to import the ipython.py module from parameterized.py (cannot import name 'depends' from partially initialized module 'param.depends' (most likely due to a circular import) (/Users/mliquet/dev/param/param/depends.py)) and the second one being catching the error with a bare try/except.

To fix this I had to move register_display_accessor to another module, that I called display.py but maybe there's a better name like hooks.py? I also inlined the imports required by the IPythonDisplay class.

To test this I had to refactor a little parameterized.py so it relies on a helper to find out whether we're withing an IPython context. My intention was to add a test to mock the IPython context, I managed to write a test that worked on its own but didn't manage in a reasonable amount of time to get it to work with the other tests. Posting that test here in case it's useful in the future:

import param
from param.display import _display_accessors

def test_ipython_autoloaded():
    with patch('param._utils._in_ipython') as mock_in_python:
        mock_in_python.return_value = True
        importlib.reload(param.parameterized)
        assert param.parameterized.param_pager is not None
        assert '_ipython_display_' in _display_accessors

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Even if I have submitted this fix, I'm not a big fan of having the IPython display hook automatically set up on import param and would rather have Param users explicitly call a function to register it.