Let's break the question into two steps.
- Can I create [and distribute] Python software that will dynamically link against a closed-source library that has a C API (the Python interpreter is not copyleft).
This should cause no problem.
The fact that the software is designed to be linked to some closed-source library, imposes no constraint on it.
Your software is your software. You may give it any license you like, and
distribute it as you want.
- and will also utilize a GPL licensed Python library?
This step is slightly more complex.
Using this library means that the combination of your software, closed-source library, and the GPL licensed Python library, means that the composite of all three must, if distributed, be licensed under GPL (some may disagree, but let's for the sake of argument assume that this is effect of the GNU GPL).
This clearly rules out distributing a project where all three components are included, as the license of GNU GPL library is incompatible with including a closed source library.
However, the fact that your software is written for the specific purpose of making all these (license-wise incompatible) components interoperate cannot impose any restrictions on how you choose to use your creative powers.
I.e. the mere existence of a legal tool know as the "GNU GPL" cannot in any way constrain what you create.
The GNU GPL can impose terms on what you distribute. If what you distribute includes a component licensed under the GNU GPL, then you're bound by the terms of the license.
This also means that if you don't distribute any component licensed under the GNU GPL, then you're not bound by the terms of the license.
So: Provided you don't distribute anything else along with your Python software, but just provide instructions to your downstream users about how to find (buy?) the closed-source library, and the GPL licensed Python library, and the steps they need to go through to assemble the these components into a working composite, then you don't have to care about the GPL.
Now, what about your users? Given the scenario described above, they will now be able to take your software, separately acquire the closed-source library, and the GPL licensed Python library, and take the steps required to assemble the complete project into a functional piece of software.
Are the users breaking the GPL? No, not as long as they don't distribute the completed project. The private use exception of the GNU GPL (and copyright law) makes the user's use of all the components legal.
Are you subverting the GNU GPL by doing it this way? IMHO, no. The GNU GPL is essentially about freedom. The GNU GPL was not written to restrict people from using closed source software in their projects.