The newly introduced Rest API does not include the ability to fetch video data.
The newly introduced Rest API is currently incorporating the functionality that before its introduction was available using the Configuration API.
For fetching live video data you need to use Image Server protocol. The sample TCPVideoViewer uses protocol methods and can be translated in to Python.
https://doc.developer.milestonesys.com/html/index.html?base=gettingstarted/intro_soap_protocols.html&tree=tree_3.html
https://doc.developer.milestonesys.com/html/index.html?base=reference/protocols/imageserver_index.html&tree=tree_3.html
https://doc.developer.milestonesys.com/html/index.html?base=samples/protocolsamples/tcpvideoviewer/readme.html&tree=tree_3.html
As you have a web application I am thinking that you will find using the Mobile SDK better and easier. Please have a look at
https://doc.developer.milestonesys.com/mipsdkmobile/