Avoid `SyntaxWarning` on Python >= 3.8
```
>>> "convolutional" == "convolutional"
True
>>> "convolutional" is "convolutional"
<stdin>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
True
```
Related to #21121
Instead of being a copy of line 76, line 79 instead correctly indicates that it will show a histogram for a gray image in curve mode, as given by the code block at line 103 referencing image "gray" instead of image "im".
Scripts for downloading models in DNN samples
* Initial commit. Utility classes and functions for downloading files
* updated download script
* Support YAML parsing, update download script and configs
* Fix problem with archived files
* fix models.yml
* Move download utilities to more appropriate place
* Fix script description
* Update README
* update utilities for broader range of files
* fix loading with no hashsum provided
* remove unnecessary import
* fix for Python2
* Add usage examples for downloadFile function
* Add more secure cache folder selection
* Remove trailing whitespaces
* Fix indentation
* Update function interface
* Change function for temp dir, change entry name in models.yml
* Update getCacheDirectory function call
* Return python implementation for cache directory selection, use more specific env variable
* Fix whitespace
[GSoC] Add siamrpnpp.py
* Updated base branch with siamrpnpp.py
* Addition of Parsers
* Merged to using few ONNX files, Changes to Parsers, Links to Repo
* Deleted whitespace
* Adjusting flake8 error
* Fixes according to review
* Fix according to review
* Addition of OpenVINO backends and Computation target devices
* Fix on backend after review
* Fixes after review
* Remove extra white space
* Removed Repeated Varaibles
* add the HH4 mode
* option to display disparity with a color map
* display current settings in the title bar
* don't close app when wanting to take screenshots
revise default proto to match the filename in documentations
fix a bug
beautify python codes
fix bug
beautify codes
add test samples with larger/smaller size
remove unless code
using bytearray without creating tmp file
remove useless codes
Improving DaSiamRPN tracker sample
* changed layerBlobs in dnn.cpp and added DaSiamRPN tracker
* Improving DaSiamRPN tracker sample
* Docs fix
* Removed outdated changes
* Trying to reinitialize tracker without reloading models. Worked with LaSOT-based benchmark with reinit rate=250 frames
* Trying to reverse changes
* Moving the model in the constructor
* Fixing some issues with names
* Variable name changed
* Reverse parser arguments changes
* Add a FLANN example showing how to search a query image in a dataset
* Clean: remove warning
* Replace dependency to boost::filesystem by calls to core/utils/filesystem
* Wait for escape key to exit
* Add an example of binary descriptors support
* Add program options for saving and loading the flann structure
* Fix warnings on Win64
* Fix warnings on 3.4 branch still relying on C++03
* Add ctor to img_info structure
* Comments modification
* * Demo file of FLANN moved and renamed
* Fix distances type when using binary vectors in the FLANN example
* Rename FLANN example file
* Remove dependency of the flann example to opencv_contrib's SURF.
* Remove mention of FLANN and other descriptors that aimed at giving hint on the other options
* Cleaner program options management
* Make waitKey usage minimal in FLANN example
* Fix the conditions order
* Use cv::Ptr
