Can't make this work on Linux Mint 19 (Ubuntu 18.04).
Compiled ReleaseMonoPy3 with XPLAT;;PYTHON3;PYTHON37;UCS4;MONO_LINUX;PYTHON_WITH_PYMALLOC.
It seems to crash on Py.GIL():

using (Py.GIL())
{
    dynamic np = Py.Import("numpy");
    Console.WriteLine(np.cos(np.pi * 2));

    dynamic sin = np.sin;
    Console.WriteLine(sin(5));

    double c = np.cos(5) + sin(5);
    Console.WriteLine(c);

    dynamic a = np.array(new List<float> { 1, 2, 3 });
    Console.WriteLine(a.dtype);

    dynamic b = np.array(new List<float> { 6, 5, 4 }, dtype: np.int32);
    Console.WriteLine(b.dtype);

    Console.WriteLine(a * b);
    Console.ReadKey();
}

Upon further investigation, it seems to crash in InitializePlatformData(), as

IntPtr platformModule = PyImport_ImportModule("platform"); // Becomes NULL pointer

Running this line immediately after setting platformModule = ...

Console.WriteLine(new PythonException().Message);

yields:

ModuleNotFoundError : No module named '_posixsubprocess'

I also tried making sure the python home and path variables are correctly set:

PythonEngine.PythonHome = @"/usr/local/bin/python3.7";
PythonEngine.PythonPath = @"/usr/local/lib/python3.7";

I have no problem importing _posixsubprocess module when running the python3.7 binary from the terminal.