input: Write a python function to read data from a CSV file using pandas.
  assert read_data('testfile1.csv').shape == (5, 3)
  assert 'column1' in read_data('testfile2.csv').columns
  assert read_data('testfile3.csv').iloc[0, 0] == 'expected_value'
output: def read_data(filename):
      import pandas as pd
      return pd.read_csv(filename)