Declare setuptools as the build backend in pyproject.toml (#350)

pyproject.toml

diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..eb62c16
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,3 @@
+[build-system]
+requires = ["setuptools >= 42.0.0"]
+build-backend = "setuptools.build_meta"