Fix a memory leak when creating Python3 modules.

Author: T045T

include/pybind11/pybind11.h

@@ -795,11 +795,17 @@ class module : public object {
         if (!options::show_user_defined_docstrings()) doc = nullptr;
 #if PY_MAJOR_VERSION >= 3
         PyModuleDef *def = new PyModuleDef();
+        // Ensure that Python does not try to delete this.
+        Py_INCREF(def);
         std::memset(def, 0, sizeof(PyModuleDef));
         def->m_name = name;
         def->m_doc = doc;
         def->m_size = -1;
-        Py_INCREF(def);
+        def->m_free = [](void* module ) {
+            if (module != NULL) {
+                delete PyModule_GetDef((PyObject*) module);
+            }
+        };
         m_ptr = PyModule_Create(def);
 #else
         m_ptr = Py_InitModule3(name, nullptr, doc);