Pytest-yapf3

- Set pytest limit to `>=7`.
- Fix a pytest warning with `from_parent`.

- Set pytest limit to `>=5.4,<8`.
- Upgrade dependencies in Pipfile and its lock.
- Enable GitHub actions.
- Delete `.travis.yml` and `.appveyor.yml`

- Fix cache missing.

- Upgrade pytest to 5.4+, fix a warning.
- Support python 3.6+ only, abandon 3.5
- Use Pipfile with `pipenv`

- Fix coverage missing lines.
- 100% test coverage.
- Fix a crash when yapf crash.
- Change development status to stable.

- Skip running if a file is not changed.
- Add more usage description in README.