How to check if video has been deleted or removed in youtube using python

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I assume there is a method to set the default version to 3.7.6 so that running any scripts I can simply do this ..

Yes can set alias or mess with update-alternatives link.
Or just use python3.7 my_script.py to run and install with eg pip3.7 install requests.
I use pyenv Simple Python Version Management then this done automatically to chosen version.