Add pyproject.toml to help dependency managers (#685)

Some dependency managers are able to use the information provided by the
`pyproject.toml` file.

It helps to provide install/build-time dependencies information, which is
currently the case with C/Cython-related packages.

python/pyproject.toml

@@ -0,0 +1,2 @@
+[build-system]
+requires = ["cython", "numpy", "pystan"]