Drop support for python 2.7 and python 3.5

[rotu]

As of this commit, pymavlink requires Python 3.6 or newer.

With the recent breaking of setup-python on github ci, this is as good a time as any to drop nominal support for old Python versions. As of today, Python 3.7 is end-of-life, but Ardupilot maintains support for Ubuntu Bionic (18.04)., which uses Python 3.6.

See #556