Fix problem where poetry won't install debugpy

debugpy is an indirect dependency of this project. When poetry
1.4.1 is used with the default configration, poetry refuses to
install debugpy, due to recognizing something wrong with the wheel.

This is the same issue (with the same dependency) as:

python-poetry/poetry#7686

For now, I'm setting installer.modern-installation to false, per
the workaround in:

python-poetry/poetry#7686 (comment)

poetry.toml

@@ -0,0 +1,2 @@
+[installer]
+modern-installation = false