_get_scheduler() exception if dask.multiprocessing missing

[johnomotani]

These lines were recently changed in #3358

xarray/xarray/backends/locks.py

Lines 87 to 92 in 3f9069b

	pass
	if actual_get is dask.multiprocessing.get:
	return "multiprocessing"
	else:
	return "threaded"

If the 'cloudpickle' package is not installed, then dask.multiprocessing is not available. The try/except that used to be wrapped around if actual_get is dask.multiprocessing.get meant that _get_scheduler() worked in that case, returning "threaded" (I assume this was the expected behaviour). After #3358, _get_scheduler() raised an AttributeError: module 'dask' has no attribute 'multiprocessing' until I installed 'cloudpickle'.

Suggest either reverting the changes that removed the try/except or making 'cloudpickle' a dependency.

---

To reproduce:

1. check 'cloudpickle' is not installed, but 'dask' is
2. execute the following commands

>>> import xarray
>>> xarray.backends.api._get_scheduler()

Expected result: "threaded"
Actual result:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-2-20da238796b7> in <module>
----> 1 xarray.backends.api._get_scheduler()

~/.local/lib/python3.6/site-packages/xarray/backends/locks.py in _get_scheduler(get, collection)
     87         pass
     88 
---> 89     if actual_get is dask.multiprocessing.get:
     90         return "multiprocessing"
     91     else:

AttributeError: module 'dask' has no attribute 'multiprocessing'