Fix pip install <local directory>

Installing the local directory from other projects using `pip` fails:
```sh
$ cd chws_tools
$ pip install ../east_asian_spacing
...
ModuleNotFoundError: No module named 'setuptools'
```

This change can work around the problem as described at:
  python-poetry/poetry#3001

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires = ["poetry-core>=1.0.0"]
+requires = ["poetry-core>=1.0.0", "setuptools"]
 build-backend = "poetry.core.masonry.api"
 
 [tool.poetry]