Move pypy3.6 to github actions

[xuhdev]

Specifying a pypy version was recently supported:
actions/setup-python#168

Move away from GitLab CI due to #81

Close #81

---

Checklist:

• Does this pull request close an issue? We encourage you to open an issue first if this pull request (PR) is not a
  minor change.
  • No
    • The change in this pull request is minor.
  • Yes
    • Close GitLab CI doesn't run when a pull request is from a fork #81

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[xuhdev]

Still really slow though.

[xuhdev]

Close for now.