chore: simplify numpy dtype ctor (#3869)

* Simplify numpy dtype ctor

* Simplify c_str ctor

* Remove compat macro for str in numpy

Author: Skylion007

include/pybind11/numpy.h

@@ -540,18 +540,16 @@ class dtype : public object {
     PYBIND11_OBJECT_DEFAULT(dtype, object, detail::npy_api::get().PyArrayDescr_Check_);
 
     explicit dtype(const buffer_info &info) {
-        dtype descr(_dtype_from_pep3118()(PYBIND11_STR_TYPE(info.format)));
+        dtype descr(_dtype_from_pep3118()(pybind11::str(info.format)));
         // If info.itemsize == 0, use the value calculated from the format string
         m_ptr = descr.strip_padding(info.itemsize != 0 ? info.itemsize : descr.itemsize())
                     .release()
                     .ptr();
     }
 
-    explicit dtype(const std::string &format) {
-        m_ptr = from_args(pybind11::str(format)).release().ptr();
-    }
+    explicit dtype(const std::string &format) : dtype(from_args(pybind11::str(format))) {}
 
-    explicit dtype(const char *format) : dtype(std::string(format)) {}
+    explicit dtype(const char *format) : dtype(from_args(pybind11::str(format))) {}
 
     dtype(list names, list formats, list offsets, ssize_t itemsize) {
         dict args;
@@ -638,7 +636,7 @@ class dtype : public object {
         }
 
         struct field_descr {
-            PYBIND11_STR_TYPE name;
+            pybind11::str name;
             object format;
             pybind11::int_ offset;
         };
@@ -653,7 +651,7 @@ class dtype : public object {
                 continue;
             }
             field_descriptors.push_back(
-                {(PYBIND11_STR_TYPE) name, format.strip_padding(format.itemsize()), offset});
+                {(pybind11::str) name, format.strip_padding(format.itemsize()), offset});
         }
 
         std::sort(field_descriptors.begin(),
@@ -1359,7 +1357,7 @@ PYBIND11_NOINLINE void register_structured_dtype(any_container<field_descriptor>
             pybind11_fail(std::string("NumPy: unsupported field dtype: `") + field.name + "` @ "
                           + tinfo.name());
         }
-        names.append(PYBIND11_STR_TYPE(field.name));
+        names.append(pybind11::str(field.name));
         formats.append(field.descr);
         offsets.append(pybind11::int_(field.offset));
     }