Update to new Stretch port names

[WasabiFan]

As mentioned in #339, motor port names have changed in Stretch. We're going to need to update the library and test infrastructure.

[dwalton76]

@WasabiFan I think we can close this one?

[WasabiFan]

I think (?) my intent was to backport this to the 1.x branch: https://github.com/ev3dev/ev3dev-lang-python/commits/ev3dev-stretch-1.x

I can't tell if I did that though. That being said, no one's complaining, so stuff seems to work...