Add yapf linter step and make CI explicit

[jaeyoo]

Fix #44

Before this PR, CI only showed two steps only in this Conversation Tab.

Continuous Integration / Checks (pull_request) Failing after 5s 
cla/google Successful in 1s — ✅ All contributors are covered under a CLA with Google

This made us frustrated to check the real cause of the error because it's implicit. So, this PR makes the CI steps explicit in this Conversation tab, which also enables the parallel executions of the whole CI steps.

Also, it adds yapf linter

[farice]

omg you're so fast that I got whiplash!

would it make sense to fix whatever errors yapf raises in the same PR?

[jaeyoo]

Since there are so many lint errors, let me open a new PR to deal with it.

[farice]

sounds great, thanks for fighting the beast! looking forward to seeing the green checkmark! 💪

[zaqqwerty]

To ease the burden, is there a way to add an excludelist that initially has all the files, then we remove them one-by-one in PRs? (new files would not be added to the list)

[jaeyoo]

To ease the burden, is there a way to add an excludelist that initially has all the files, then we remove them one-by-one in PRs? (new files would not be added to the list)

That's a good idea. let me add the excludelist for linter, and defeat the lint errors one-by-one :)

[jaeyoo]

After PyPI recently changed their client webauth scheme (1 hr ago), poetry can't find packages for now. Let's wait for their patch...

[jaeyoo]

Done. may I merge this? @zaqqwerty

[zaqqwerty]

Thanks Jae, LGTM