Add 2-line setup.py for setuptools packaging

This commit adds a 2-line (3-line, if you count the blank line)
`setup.py` script that provides the bare minimum needed to support
packaging with `setuptools`, e.g.:

```shell
python3 setup.py sdist bdist_wheel
```

This basic `setup.py` should also support editable installations,
as requested in nschloe#1223.

The basic rationale for `setuptools` packaging is for high-performance
computing sites that lag behind in software tooling. While projects
like Spack help update this tooling to permit updating sometimes
ancient toolchains, HPC site policies -- including the HPC site I
work at -- sometimes require repackaging Python packages from source
for internal approval and security auditing, as well as for portable
installation to air-gapped machines.

While I completely agree that `setup.py` is a relic -- and to that
effect, just spent a few hours adding source code and binary wheel
installations of `flit_core`, `flit`, `pep517`, and `build` to my HPC
site for approval -- it's much, much easier to point to upstream code
than it is to have to figure out that these lines must be added for
bespoke packaging, add them, look through project documentation like
`CONTRIBUTING.md`, discover through looking at pull requests and
issues on GitHub that such a pull request may not be accepted, and
then figure out how to pass that information on to security auditors
and colleagues.

If this solution is unacceptable, I welcome other suggestions,
including writing a statement to the effect of "Pull requests adding a
`setup.py` file will not be accepted to this repository" somewhere in
documentation, along with the suggested solution of adding

```python
from setuptools import setup

setup()
```

or similar to a local copy of the repository so as to avoid the sorts
of discussions seen in nschloe#1224.

setup.py

@@ -0,0 +1,3 @@
+from setuptools import setup
+
+setup()