input: Write a python function to count the number of non-null values in each column of a DataFrame.
  assert countValues(pd.DataFrame({'A': [1, 2, None, 4], 'B': [None, 2, 3, 4]})) == {'A': 3, 'B': 3}
  assert countValues(pd.DataFrame({'A': [None, None, None], 'B': [1, 2, 3]})) == {'A': 0, 'B': 3}
  assert countValues(pd.DataFrame({'A': [1, 2], 'B': [3, 4], 'C': [None, 5]})) == {'A': 2, 'B': 2, 'C': 1}
output: def countValues(df):
      return df.notnull().sum().to_dict()