Fix sklearn dep warning

[r9y9]

Fixes #99

[codecov-io]

Codecov Report

Merging #100 into master will decrease coverage by 0.06%.
The diff coverage is 70.00%.

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
- Coverage   80.48%   80.41%   -0.07%     
==========================================
  Files          32       32              
  Lines        1860     1869       +9     
==========================================
+ Hits         1497     1503       +6     
- Misses        363      366       +3     

Impacted Files	Coverage Δ
nnmnkwii/preprocessing/generic.py	98.00% <70.00%> (-1.47%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fb7c278...63f351d. Read the comment docs.