inconsistent numpy dtype in python and C++

Hi,
I'm trying to use the boost numpy bindings. If I pass two ndarray with different dtype to C++, they don't preserve the correct dtype. Here is a simple example:
dummy.cpp:
```
#include
#include
#include

namespace py = boost::python;
namespace np = boost::python::numpy;

void dummy(np::ndarray const& a, np::ndarray const& b) {

char const* at = py::extract(py::str(a.get_dtype()));
char const* bt = py::extract(py::str(b.get_dtype()));

std::cout << at << std::endl;
std::cout << bt << std::endl;
}

BOOST_PYTHON_MODULE(dummy) {
Py_Initialize(); // need initialize in BOOST_PYTHON_MODULE, or will get segmentation fault and core dump
np::initialize();
py::def("dummy", dummy);
}
```
compile
```
g++ -lboost_python38 -lboost_numpy38 -L$CONDA_PREFIX/lib -I$CONDA_PREFIX/include -I$CONDA_PREFIX/include/python3.8 -fPIC --shared -o dummy.so dummy.cpp
```
run_dummpy.py
```
import numpy as np
import dummy

a = np.array([[1, 2, 3], [4, 5, 6]], dtype = np.float32)
print("python", a.dtype)
b = np.array([[1, 2, 3], [4, 5, 6]], dtype = np.double)
print("python", b.dtype)

dummy.dummy(a, b)
```
The output is inconsistent. The first time I run it by ```python run_dummy.py ``` it gives
```
python float32
python float64
__str__
float64
```
If I re-run it ```python run_dummy.py ```, it gives
```
python float32
python float64
float64
float64
```
Still not the right output.
I'm using python 3.8.2, numpy 1.18.1, and boost 1.72.0. boost is installed from anaconda ```conda install -c conda-forge boost```
Could you help? Thanks