Fix setuptools warning

Fixes

```
_Warning: Package 'petab.schemas' is absent from the `packages` configuration.
```

Author: dweindl

pyproject.toml

@@ -82,6 +82,7 @@ Repository = "https://github.com/PEtab-dev/libpetab-python"
 Documentation = "https://petab.readthedocs.io/projects/libpetab-python/"
 
 [tool.setuptools.packages.find]
+where = ["petab"]
 include = ["petab", "petab.*"]
 namespaces = false