Fix sklearn version and replace it by scikit-learn

requirements/test_requirements.txt

@@ -2,7 +2,7 @@ pytest>=5.4.0, <6.0.0
 pytest-cov>=2.8.0, <3.0.0
 pytest-lazy-fixture>=0.6.0, <1.0.0
 pytest-mock>=3.1.0, <4.0.0
-sklearn>=0.23.0, <0.24.0
+scikit-learn>=0.23.0, <0.24.0
 flake8>=3.0.0, <4.0.0
 black==19.10b0  # pin black version because it is not compatible with a pip range (because of non semver version number)
 isort>=5.0.0, <6.0.0