Pin setuptools to workaround package_index removal

Fix broadinstitute#192 , which was
caused by the removal of `package_index` from setuptools in
pypa/setuptools#4963 .

Note that I needed to use `setuptools.build_meta:__legacy__` as
`build-backend` (instead of the usual `setuptools.build_meta`)
because otherwise `python3 -m build` would fail with:

```
ModuleNotFoundError: No module named 'firecloud'
```

Author: nsoranzo

pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+requires = ["setuptools<80.3.0"]
+build-backend = "setuptools.build_meta:__legacy__"