transform: Fix scikit-learn 0.21.0 incompatibility

The commit 2bd87f6ed8d36f89cab73552665e4e59e892262b from scikit-learn
removed the sklearn.utils.fixes.signature function in favour of using
the inspect.signature function.

seglearn/transform.py

@@ -8,7 +8,7 @@
 from scipy.interpolate import interp1d
 from sklearn.base import BaseEstimator, TransformerMixin
 from sklearn.utils import check_random_state, check_array, check_consistent_length, shuffle
-from sklearn.utils.fixes import signature
+from inspect import signature
 from sklearn.exceptions import NotFittedError
 from sklearn.utils import check_random_state, check_array, check_consistent_length
 from sklearn.utils.metaestimators import _BaseComposition