input: def generate_book_table():
      """Generate an HTML table displaying information about the top 10 bestselling books in the 'Fantasy' genre.
      The table will have columns for Title, Author, Genre, and Sales.
      The table is generated by querying a SQLite database named 'books.db'.
      >>> generate_book_table()
      '<table><tr><th>Title</th><th>Author</th><th>Genre</th><th>Sales</th></tr><tr><td>Book1</td><td>Author1</td><td>Fantasy</td><td>100000</td></tr><tr><td>Book2</td><td>Author2</td><td>Fantasy</td><td>90000</td></tr><tr><td>Book3</td><td>Author3</td><td>Fantasy</td><td>80000</td></tr><tr><td>Book4</td><td>Author4</td><td>Fantasy</td><td>70000</td></tr><tr><td>Book5</td><td>Author5</td><td>Fantasy</td><td>60000</td></tr><tr><td>Book6</td><td>Author6</td><td>Fantasy</td><td>50000</td></tr><tr><td>Book7</td><td>Author7</td><td>Fantasy</td><td>40000</td></tr><tr><td>Book8</td><td>Author8</td><td>Fantasy</td><td>30000</td></tr><tr><td>Book9</td><td>Author9</td><td>Fantasy</td><td>20000</td></tr><tr><td>Book10</td><td>Author10</td><td>Fantasy</td><td>10000</td></tr></table>'
      """
output:     import sqlite3
      conn = sqlite3.connect('books.db')
      cursor = conn.cursor()
      cursor.execute("SELECT Title, Author, Genre, Sales FROM books WHERE Genre='Fantasy' ORDER BY Sales DESC LIMIT 10")
      rows = cursor.fetchall()
      conn.close()
  
      table = '<table><tr><th>Title</th><th>Author</th><th>Genre</th><th>Sales</th></tr>'
      for row in rows:
          table += f'<tr><td>{row[0]}</td><td>{row[1]}</td><td>{row[2]}</td><td>{row[3]}</td></tr>'
      table += '</table>'
      return table