Fix cylic import artifical data.

[ericpre]

As noticed in conda-forge/hyperspy-feedstock@951ad46, there is a cyclic import in hyperspy.datasets.artificial_data which breaks the import of hyperspy._lazy_signals.

Traceback (most recent call last):

  File "<ipython-input-1-f66322193b6e>", line 1, in <module>
    import hyperspy._lazy_signals

  File "/home/eric/Dev/hyperspy/hyperspy/_lazy_signals.py", line 6, in <module>
    from hyperspy._signals.eds_sem import LazyEDSSEMSpectrum

  File "/home/eric/Dev/hyperspy/hyperspy/_signals/eds_sem.py", line 23, in <module>
    from hyperspy._signals.eds import (EDSSpectrum, LazyEDSSpectrum)

  File "/home/eric/Dev/hyperspy/hyperspy/_signals/eds.py", line 27, in <module>
    from hyperspy import utils

  File "/home/eric/Dev/hyperspy/hyperspy/utils/__init__.py", line 24, in <module>
    import hyperspy.datasets.example_signals

  File "/home/eric/Dev/hyperspy/hyperspy/datasets/__init__.py", line 21, in <module>
    from hyperspy.datasets import artificial_data

  File "/home/eric/Dev/hyperspy/hyperspy/datasets/artificial_data.py", line 9, in <module>
    from hyperspy.signals import EELSSpectrum, Signal2D

  File "/home/eric/Dev/hyperspy/hyperspy/signals.py", line 46, in <module>
    from hyperspy._signals.eds_sem import EDSSEMSpectrum

ImportError: cannot import name 'EDSSEMSpectrum' from 'hyperspy._signals.eds_sem' (/home/eric/Dev/hyperspy/hyperspy/_signals/eds_sem.py)