Fix sklearn FutureWarning for params not available for cloning.

[tomolopolis]

using CRF within a sklearn Pipeline, raises a FutureWarning with sklearn 0.23. This will raise an AttributeError from version 0.24.

Repro for the warning.

from sklearn.pipeline import Pipeline
from sklearn_crfsuite import CRF
from sklearn.preprocessing import StandardScaler
Pipeline([('scaler', StandardScaler()), ('crf', CRF())]).get_params()
/Users/tom/anaconda3/envs/cattrainer/lib/python3.7/site-packages/sklearn/base.py:213: FutureWarning: From version 0.24, get_params will raise an AttributeError if a parameter cannot be retrieved as an instance attribute. Previously it would return None.
  FutureWarning)

Tested in jupyter with this change via pip install -e sklearn-crfsuite/. and the proposed change. tox passes.

[codecov-commenter]

Codecov Report

Merging #57 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
+ Coverage   92.60%   92.66%   +0.05%     
==========================================
  Files           8        8              
  Lines         257      259       +2     
  Branches       37       37              
==========================================
+ Hits          238      240       +2     
  Misses         15       15              
  Partials        4        4              

Impacted Files	Coverage Δ
sklearn_crfsuite/estimator.py	95.52% <100.00%> (+0.06%)	⬆️

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