input: Write a python function to get the closing price of a stock for the past month using yfinance.
  assert get_closing_price("AAPL", "abc123") > 0
  assert isinstance(get_closing_price("MSFT", "abc123"), float)
  assert get_closing_price("GOOGL", "abc123") != 0
output: import yfinance as yf
  from datetime import datetime, timedelta
  
  def get_closing_price(ticker, api_key):
      end_date = datetime.now()
      start_date = end_date - timedelta(days=30)
      stock_data = yf.download(ticker, start=start_date, end=end_date)
      closing_price = stock_data['Close'].iloc[-1]
      return closing_price