Ouranosinc / xscen / 13187369786

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Drop setuptools, use flit (#519)

### What kind of change does this PR introduce?

* Drops `setuptools` build backend for `flit`.
* Moves the `requirements_upstream.txt` file to CI folder.
* Removes the obsolete `Manifest.in`.

### Does this PR introduce a breaking change?

Yes. `setuptools` is no longer used to build the package. Build
requirements are much simpler.

### Other information:

[`flit`](https://flit.pypa.io/en/stable/) is the backend used by the
cookiecutter and most Ouranos projects. It is much simpler in its
configuration and design. We don't compile `xscen` nor do we manage
dependencies using lock files (like `poetry` does).

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