numpy 2.x scalar types np.float32 and others are no longer convertible

## Summary

In NumPy 2.0, scalar types such as `np.float32`, `np.int32`, and `np.int64` no longer subclass Python's built-in `float` or `int` (per
[NEP 51](https://numpy.org/neps/nep-0051-scalar-representation.html)). Boost.Python's built-in rvalue converters for C++ `double`, `float`, `int`, and `long` use `PyFloat_Check` / `PyLong_Check`, which return false for these NumPy scalars. The result is `ArgumentError: Python argument types did not match C++ signature` for any Boost.Python-wrapped function that previously accepted NumPy scalars via implicit conversion.

`np.float64` is unaffected because it still subclasses Python `float`.

## Reproducing

Put the following in `test_module.cpp`:
```
#include

double take_double(double x) { return x; }
float take_float(float x) { return x; }
int take_int(int x) { return x; }
long take_long(long x) { return x; }

BOOST_PYTHON_MODULE(test_module)
{
using namespace boost::python;
def("take_double", take_double);
def("take_float", take_float);
def("take_int", take_int);
def("take_long", take_long);
}
```
Then do:
```
wget https://repo.anaconda.com/miniconda/Miniconda3-latest-Linux-x86_64.sh
bash Miniconda3-latest-Linux-x86_64.sh -b -p $PWD/mc3
source mc3/etc/profile.d/conda.sh
conda create -y -p $PWD/env -c conda-forge python=3.11 "numpy>=2.0" libboost-python-devel gxx_linux-64
conda activate $PWD/env
$CXX -shared -fPIC -o test_module.so test_module.cpp -Ienv/include/python3.11 -Ienv/include -Lenv/lib -lboost_python311 -Wl,-rpath,env/lib
python -c "import numpy as np; import test_module; test_module.take_double(np.float32(1.5))"
```

The python script at the bottom of the issue gives some more types that don't convert.

## Workaround

Users can explicitly convert before passing to Boost.Python:

```python
func(float(np.float32(1.5))) # works
func(int(np.int32(42))) # works
func(np.float32(1.5).item()) # also works
```
I have implemented this in a few places and I'm sure many others have as well. See https://github.com/cctbx/cctbx_project/issues/1084

## Longer python reproducer
```
import numpy as np
import test_module

# Show which numpy scalars still subclass builtins (the root cause)
for np_type in [np.float64, np.float32, np.float16,
np.int64, np.int32, np.int16, np.int8]:
builtin = float if np.issubdtype(np_type, np.floating) else int
print("issubclass(%s, %s): %s" % (
np_type.__name__, builtin.__name__, issubclass(np_type, builtin)))
print()

scalar_values = [
np.float64(1.5),
np.float32(1.5),
np.float16(1.5),
np.int64(42),
np.int32(42),
np.int16(42),
np.int8(42),
np.uint64(42),
np.uint32(42),
]

functions = [
(test_module.take_double, "take_double"),
(test_module.take_float, "take_float"),
(test_module.take_int, "take_int"),
(test_module.take_long, "take_long"),
]

for func, func_name in functions:
for scalar in scalar_values:
try:
result = func(scalar)
print("Pass", func_name, type(scalar))
except Exception as e:
print("Fail", func_name, type(scalar))
```