input: Write a function to load data from a text file using numpy and fit a decision tree classifier from sklearn
  assert load_and_fit_tree('data.txt', y) is not None
output: def load_and_fit_tree(filename, y):
      import numpy as np
      from sklearn.tree import DecisionTreeClassifier
  
      # Load data from the text file
      X = np.loadtxt(filename)
      
      # Initialize the Decision Tree Classifier
      clf = DecisionTreeClassifier()
      
      # Fit the classifier with the data
      clf.fit(X, y)
      
      return clf