I bought the Tello, and updated it to the latest firmware: 01.04.78.01
Upon trying to instantiate the object "Tello", the __init__ method would raise a RuntimeError('Tello rejected attempt to enter command mode').
After a little debugging, I figured out that the exception was thrown because the 'OK' expected as a response upon send_command('command') (line 59) was returned in lower caps format, i.e. 'ok'.
Possibly this mismatch was due to the latest firmware update, and to have the tello.py library work again, it's simply necessary to change the 'OK' in line 59 to 'ok'.