Update version of pysdl

[krichardsson]

There is a hard requirement in (setup.py)[https://github.com/bitcraze/crazyflie-clients-python/blob/3d2229a/setup.py#L79-L80] to use pysdl2 0.9.6.
0.9.7 has been released and we should test if it works and upgrade.
Users that has 0.9.7 can not use the client.

Workaround: install pysdl2 0.9.6 or update setup.py to 0.9.7