Fix dtype string leak

`PyArray_DescrConverter_` doesn't steal a reference to the argument,
and so the passed arguments shouldn't be `.release()`d.

include/pybind11/numpy.h

@@ -440,7 +440,7 @@ class dtype : public object {
     /// This is essentially the same as calling numpy.dtype(args) in Python.
     static dtype from_args(object args) {
         PyObject *ptr = nullptr;
-        if (!detail::npy_api::get().PyArray_DescrConverter_(args.release().ptr(), &ptr) || !ptr)
+        if (!detail::npy_api::get().PyArray_DescrConverter_(args.ptr(), &ptr) || !ptr)
             throw error_already_set();
         return reinterpret_steal<dtype>(ptr);
     }

tests/test_numpy_dtypes.cpp

@@ -448,4 +448,7 @@ TEST_SUBMODULE(numpy_dtypes, m) {
 
     // test_register_dtype
     m.def("register_dtype", []() { PYBIND11_NUMPY_DTYPE(SimpleStruct, bool_, uint_, float_, ldbl_); });
+
+    // test_str_leak
+    m.def("dtype_wrapper", [](py::object d) { return py::dtype::from_args(std::move(d)); });
 }

tests/test_numpy_dtypes.py

@@ -293,6 +293,19 @@ def test_register_dtype():
     assert 'dtype is already registered' in str(excinfo.value)
 
 
+@pytest.unsupported_on_pypy
+def test_str_leak():
+    from sys import getrefcount
+    fmt = "f4"
+    pytest.gc_collect()
+    start = getrefcount(fmt)
+    d = m.dtype_wrapper(fmt)
+    assert d is np.dtype("f4")
+    del d
+    pytest.gc_collect()
+    assert getrefcount(fmt) == start
+
+
 @pytest.requires_numpy
 def test_compare_buffer_info():
     assert all(m.compare_buffer_info())