requirements.txt has pymavlink pinned to 2.0.6 instead of 2.0.8

[cclauss]

pymavlink 2.0.8 opens up the door to working on Python3.

This is part of PR #673 but could also be done separately.

[neatorobito]

At this time of writing, it should be updated to 2.1.0 in order to include support for the new copter mode GUIDED_NOGPS.

[cclauss]

https://github.com/dronekit/dronekit-python/blob/master/requirements.txt#L1