python 2->3

[davidhyman]

according to futurize
+ some minor fixes

Really we'll need something like workflows on circleci 2 to check the full test suite against Python3. Could do that as a separate ticket, or the same.

[davidhyman]

ref #601

[theotherjimmy]

@davidhyman I would like to have the workflows as part of this PR please. We may need to block this on support for mbed compile, mbed test and mbed export

[screamerbg]

Looks great! Thanks @davidhyman. This would have to wait until the other mbed OS tools need to support Python3 first as @theotherjimmy explained.

[cmonr]

@theotherjimmy @screamerbg Once this (ARMmbed/mbed-os#6592) goes in, would anyone be opposed to moving this PR along?

[screamerbg]

@davidhyman Could you please rebase this PR?

[cmonr]

@davidhyman I wanted to check something with this PR.

The goal should be to add compatibility to make sure that mbed-cli can run with either Py2 or Py3. Does this PR do that? If so, would you mind changing the PR title to reflect this? (python 2->3 to python 2 & 3)

[screamerbg]

@davidhyman Could you please rebase this PR on top of master?

[cmonr]

Closing due to inactivity. Changes will be integrated into a separate PR for review.