input: Write a python function to calculate the sum of a specific column from a SQL table using pandas.
  assert sum_column_from_sql("SELECT * FROM table", "column", conn) == expected_sum
  assert sum_column_from_sql("SELECT * FROM another_table", "another_column", conn) == another_expected_sum
  assert sum_column_from_sql("SELECT * FROM third_table", "third_column", conn) == third_expected_sum
output: def sum_column_from_sql(query, column, conn):
      import pandas as pd
      df = pd.read_sql_query(query, conn)
      return df[column].sum()