Ensure backend is initialized on import

[philippjfr]

• Fixes Geoviews should run the backend change hook when imported #273

[TimoRoth]

I'm getting errors after upgrading to 1.6.1 due to this:

Traceback (most recent call last):
  File "./app.py", line 18, in <module>
    import geoviews as gv
  File "/home/users/timo/oggm_venv/lib/python3.7/site-packages/geoviews/__init__.py", line 30, in <module>
    if extension._loaded:
AttributeError: type object 'extension' has no attribute '_loaded'

frozen environment: https://bpaste.net/show/70053f841805

[philippjfr]

@TimoRoth You will have to upgrade holoviews to a minimum of 1.11.0 and pyviz_comms to a minimum of 0.7.0.

[philippjfr]

Hmm, your environment seems to indicate you have those versions. Not quite sure how this could happen with those versions but I'll have a look.

[philippjfr]

Okay nevermind, I can see how it can happen. I'll push a fix shortly and then tag 1.6.2.