It looks like the shebang is not replaced with the correct one. Yes you need to change it to python3.
Inside a CMK 2.0 site only python3 is available.
For the “pip3 install …” @r.sander was completely right that the requests is already included. Only if you need modules that are not already existing, then you need to install it with “pip3 install …”.
If this is changed and you start the script directly inside your site, you should get the message Webhook URL not set.