Sandbox-api

* `Sandbox.check()` now return the size of the environment / file, 0 if not
found. It should not break backward compatibility because of how int are
evaluated as boolean.
* Added an `async` implementation of the api using the class `ASandbox`.
* `async` implementation depends on `aiohttp`.
* Drop support for Python 3.5 & 3.6

* Update README.md

* Now compatible with Sandbox 3.0.0

* Now use Github Action for testing and deployment to PyPI

* Added method `Sandbox451`
* Added python 3.8 tp test matrix

* Added method `check()` to `Sandbox` to check if an environment/file exists on the sandbox.