input: Write a python function to create and train a decision tree classifier using scikit-learn.
  assert clf.predict([[a, b, c, d, e, f, g]]) in [0, 1]
  assert clf.predict([[h, i, j, k, l, m, n]]) in [0, 1]
  assert clf.predict([[o, p, q, r, s, t, u]]) in [0, 1]
output: def train_decision_tree_classifier(X, y):
      from sklearn.tree import DecisionTreeClassifier
      clf = DecisionTreeClassifier()
      clf.fit(X, y)
      return clf