Fix AttributeError in test_naive_bayes.py
          #91
        
          
      
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Fixed an AttributeError in test_naive_bayes.py caused by trying to access the non-existent sigma_ attribute of the GaussianNB object from sklearn.
In more recent versions of sklearn, the var_ attribute should be used to obtain the variances for each class, instead of sigma_. This change replaces all references of sklearn_NB.sigma_ with sklearn_NB.var_ to ensure the code is compatible with the current sklearn API.
This modification ensures the correct execution of the tests and aligns with the latest version of sklearn.