Merge pull request #17436 from vpisarev:fix_python_io

* fixed #17044
1. fixed Python part of the tutorial about using OpenCV XML-YAML-JSON I/O functionality from C++ and Python.
2. added startWriteStruct() and endWriteStruct() methods to FileStorage
3. modifed FileStorage::write() methods to make them work well inside sequences, not only mappings.

* try to fix the doc builder

* added Python regression test for FileStorage I/O API ([TODO] iterating through long sequences can be very slow)

* fixed yaml testing

doc/tutorials/core/file_input_output_with_xml_yml/file_input_output_with_xml_yml.markdown

@@ -107,8 +107,9 @@ you may access it. For sequences you need to go through them to query a specific
     then we have to specify if our output is either a sequence or map.
 
     For sequence before the first element print the "[" character and after the last one the "]"
-    character. With Python, the "]" character could be written with the name of the sequence or
-    the last element of the sequence depending on the number of elements:
+    character. With Python, call `FileStorage.startWriteStruct(structure_name, struct_type)`,
+    where `struct_type` is `cv2.FileNode_MAP` or `cv2.FileNode_SEQ` to start writing the structure.
+    Call `FileStorage.endWriteStruct()` to finish the structure:
     @add_toggle_cpp
     @snippet cpp/tutorial_code/core/file_input_output/file_input_output.cpp writeStr
     @end_toggle