input: Write a python function to get current temperature of a given city using OpenWeatherMap API.
  assert get_temperature("London") is not None
  assert isinstance(get_temperature("New York"), float)
  assert get_temperature("NonExistentCity") is None
output: import requests
  
  def get_temperature(city):
      api_key = 'your_api_key_here'
      base_url = 'http://api.openweathermap.org/data/2.5/weather'
      params = {'q': city, 'appid': api_key, 'units': 'metric'}
      response = requests.get(base_url, params=params)
  
      if response.status_code == 200:
          data = response.json()
          return data.get('main', {}).get('temp')
      else:
          return None