If you're a Ruby application, sure, you can use a Python library by forking off a subprocess.
Just make sure to document the installation requirements - in addition to having Ruby and the correct gems installed, you also need Python (which version?) and the correct pip libraries installed.
If you're a Ruby library, instead of an application, forking off a Python process is a nonstarter, unless you want to propagate those requirements out to every single application using your library.
I'm mostly thinking that if you already have a Python library you really like, chances are you already have a Python environment capable of running it. (The environment doesn't have to be related to your Ruby environment at all! If you want to get Ruby through your OS and Python through a Docker container, or whatever, that should work fine.)
It's fairly rare that you actually need to be in the same process. (The thing I wrote was a wrapper for unshare(), so it did strictly need to be in-process, but that's an unusual use case.) Even if you want to share large amounts of data between your process and the library, you can often use things like memory-mapped files.
And there are some benefits from splitting up the languages, including the ability to let each language's runtime handle parallelism (finding out that the C library you want to use isn't actually thread-safe is no fun!), letting each language do exception-handling on its own, increased testability, etc.