Raise a clear error when play_tune is used over MAVLink 1

[Abhayindia]

Third fix out of the #1255 SITL sweep, after #1256 and #1257.

vehicle.play_tune() fails against any MAVLink 1 link with a confusing error that points at an unrelated message:

AttributeError: 'MAVLink' object has no attribute 'play_tune_encode'. Did you mean: 'altitude_encode'?

PLAY_TUNE exists only in the MAVLink 2 dialects. I checked all four:

dialect	play_tune_encode
v20.ardupilotmega / v20.common	present
v10.ardupilotmega / v10.common	absent

Vehicle.message_factory is self._master.mav, bound to whatever dialect the link negotiates, and the APM:Copter 3.3 binary dronekit-sitl ships is from 2015, so the link comes up as pymavlink.dialects.v10.ardupilotmega. There is no MAVLink 1 equivalent to fall back to, so this raises an APIException saying why instead.

Verified against dronekit-sitl copter. Before, the example dies on the AttributeError above. After:

dronekit.APIException: play_tune requires MAVLink 2. This connection negotiated
MAVLink 1, whose dialects have no PLAY_TUNE message, so the tune cannot be sent.

One thing I deliberately left out of the message: I tried MAVLINK20=1 to force the newer dialect, and connect(..., wait_ready=True) then hangs until timeout against this SITL, so suggesting it would have sent people down a dead end.

Based on master, independent of #1256 and #1257. Note that #1253 has to be applied before any example can run at all, since import dronekit currently fails with ModuleNotFoundError: No module named 'past'; I tested this change with that fix applied locally.