This is has been around for a long time.
Its done client side, meaning you can load the page, disconnect from the internet, and generate the key pair.
You probably already know this but I mention just in case:
There maybe a newer solution out there for Vouchers. Other people will probably chime in with the new hotness.
And for better security, load the page locally, and not from that website:
Here's a way to create a paper wallet from python alone:
For bulk paper wallets, here's python to do that:
And for those, there's the suggestion to not use pip to install, but use the source code, as mentioned here:
Nice. Local tools preferred for known reasons.