text: Title: async giving unexpected results
  Body: I'm trying to use a .csv to read ~7500 stock symbols and download the data using the alpha_vantage API.
  
  Here is an example using my naive way:
  
  ```python3
  from alpha_vantage.timeseries import TimeSeries
  import pandas as pd
  api_key = ""
  
  def get_ts(symbol):
      
      ts = TimeSeries(key=api_key, output_format='pandas')
      data, meta_data = ts.get_daily_adjusted(symbol=symbol, outputsize='full')
      fname = "./data_dump/{}_data.csv".format(symbol)
      data.to_csv(fname)
  
  symbols = ['AAPL', 'GOOG', 'TSLA', 'MSFT']
  
  for s in symbols:
      get_ts(s)
  ```
  
  Unfortunately this takes ~3 hrs using tqdm as a wrapper. So let's try to follow the article y'all wrote for async usage:
  
  ```python3
  from alpha_vantage.async_support.timeseries import TimeSeries
  import pandas as pd
  import asyncio
  
  async def get_data(symbol):
      ts = TimeSeries(key=api_key, output_format='pandas')
      
      try:
          data, _ = await ts.get_daily_adjusted(symbol=symbol, outputsize='full')
          await ts.close()
          fname = "./data_dump/{}_data.csv".format(symbol)
          data.to_csv(fname)
  
      except:
          pass
      return(None)
  
  if __name__ == "__main__":
  
      nasdaq = pd.read_csv('nasdaq_screener.csv')
      loop = asyncio.get_event_loop()
      tasks = [get_data(symbol) for symbol in nasdaq.Symbol]
      group1 = asyncio.gather(*tasks)
      results = loop.run_until_complete(group1)
      print(results)
  ```
  
  The problem is I don't really know what I'm doing with async operations in python. It works well for about 140 of the files but then just gives me this output and stops running:
  
  ```
  client_session: <aiohttp.client.ClientSession object at 0x000002395D890730>
  Unclosed client session
  client_session: <aiohttp.client.ClientSession object at 0x000002395D890CA0>
  Unclosed client session
  client_session: <aiohttp.client.ClientSession object at 0x000002395D89B250>
  Unclosed client session
  ...
  ```
  
  Any advice is appreciated! I'm not really sure if I'm handling the async api calls the right way, and I know pandas `to_csv` isn't async, but I'm trying to get the api logic right before trying anything fancier like `aiofiles`.