Continuous Integration for StackStorm Exchange packs using Python 3

[Kami]

We already have a good Continuous Integration setup in place for all the StackStorm Exchange packs which runs various checks and pack tests under Python 2.7.

We need the same thing for all the packs, but running those checks under Python 3.

To begin with, we should run those checks for all the packs to identify which packs work with Python 3 and not treat those Python 3 CI failures as fatal.

Once we identified that, we should mark those packs as Python 3 compatible (updating metadata in pack.yaml) and then only run Python 3 Exchange checks on packs which claim Python 3 compatibility.

Related PRs:

• Add support for Python 3.x checks to pack Circle CI config StackStorm-Exchange/ci#61
• Add new "python_versions" attribute to the pack metadata file #4474

[Kami]

Closing this since this has already been implemented as part of various PRs.