Yes, the Uplink is available on the Free plan!
UPDATE: Anvil now supports Python 3 in Server Modules for both free and paid plans!
Free users use the Basic Python environments (which are less less computationally expensive for us to sandbox, but lack some modules), whereas paid users can use the Full Python 2 or 3 environments.
Also, thanks for the doc request! I have now written up a “What Python versions can I use with Anvil?” page, which will be published in the next few days. You should be able to find it from Google, and you’ll definitely be able to find it from the documentation search in the Anvil editor or the Learning Centre.