input: Write a python function to read a specific column from an Excel file and iterate over its rows.
  assert read_and_iterate_column('filename.xlsx', 'column_name') == None # This assert is for checking the function runs without errors.
output: import pandas as pd
  
  def read_and_iterate_column(filename, column_name):
      df = pd.read_excel(filename)
      if column_name in df.columns:
          for value in df[column_name]:
              print(value)