convert CI from CircleCI to GHA

[cognifloyd]

part of StackStorm/community#63

this is the last pack maintained by the st2 community that uses the old CircleCI stuff from the StackStorm-Exchange/ci repo

[cognifloyd]

I also reformatted with black. I needed to make sure pylint and flake8 ran across all of the files, and that was the quickest way to do it.

There's a TODO to force check all files all the time: StackStorm-Exchange/ci#88
But I don't want to start doing that until no packs relies on CircleCI any more - including this repo.

[arm4b]

Thanks!

[arm4b]

For information, it's not a pack that is maintained by the community, but it's a pack that's used for stackstorm community and Slack.

See, for example, part that's posting in #development Slack.
https://github.com/StackStorm/stackstorm-st2community/blob/master/rules/post_daily_summary_to_slack.yaml
https://github.com/StackStorm/stackstorm-st2community/blob/master/actions/retrieve_data_and_send_daily_stats_to_slack.yaml
for example
or TSC reminder in #tsc: https://github.com/StackStorm/stackstorm-st2community/blob/master/rules/post_tsc_reminder_to_slack.yaml