fix: submodules creation and registration

- Add special case handling when submodule has the same name as parent
- `PyDict_SetItemString` doesn't steal reference, so reference count
  should be explicitly decremented to transfer object life-time
  ownership
- Add sanity checks for module registration input
- Add Python 2 and Python 3 reference counting handling

modules/core/include/opencv2/core/bindings_utils.hpp

@@ -219,6 +219,12 @@ AsyncArray testAsyncException()
     return p.getArrayResult();
 }
 
+namespace nested {
+CV_WRAP static inline bool testEchoBooleanFunction(bool flag) {
+    return flag;
+}
+} // namespace nested
+
 namespace fs {
     CV_EXPORTS_W cv::String getCacheDirectoryForDownloads();
 } // namespace fs