Feature importances for DART booster not available via sklearn interface

[Ewande]

This bug was fixed in Booster.get_score (see #4073) but it's still present in sklearn.py

xgboost/python-package/xgboost/sklearn.py

Line 539 in 0ce300e

if getattr(self, 'booster', None) is not None and self.booster != 'gbtree':

vs

xgboost/python-package/xgboost/core.py

Line 1503 in 0ce300e

if getattr(self, 'booster', None) is not None and self.booster not in {'gbtree', 'dart'}:

It would make sense to remove that condition from sklearn.py, since an exception will be raised from Booster.get_score anyway.

[hcho3]

See #4525