Added a method for converting a server dictionary to a server list

[Tivasic]

I found the following error in the code if I want to get a better server this way:

sp = speedtest.Speedtest()
best_servers = sp.get_servers()
sp.get_best_server(best_servers)

I get an error:

Traceback (most recent call last):
  File "/speedtest/main.py", line 18, in <module>
    test_connection()
  File "/speedtest/main.py", line 8, in test_connection
    sp.get_best_server(best_servers)
  File "\speedtest\venv\lib\site-packages\speedtest.py", line 1459, in get_best_server
    url = os.path.dirname(server['url'])
TypeError: 'float' object is not subscriptable

I also added the else block, otherwise the condition is incorrect on line 1456.