Adds set_name method of pybind11::capsule class

This calls PyCapsule_SetName on the underlying capsule object.

Author: oleksandr-pavlyk

include/pybind11/pytypes.h

@@ -1638,6 +1638,13 @@ class capsule : public object {
     }
 
     const char *name() const { return PyCapsule_GetName(m_ptr); }
+
+    /// Replaces a capsule's name *without* calling the destructor on the existing one.
+    void set_name(const char * &new_name) {
+        if (PyCapsule_SetName(m_ptr, new_name) != 0) {
+            throw error_already_set();
+        }
+    }
 };
 
 class tuple : public object {