Thanks for asking! From an explainer by the FSF:
[T]he GPLv2 was written when the client/server paradigm was not widespread; it could not provide the copyleft assurance desired for Affero’s platform. That is to say, one could obtain Affero’s source code, modify it, and allow users access to the program over a network without the obligation of releasing its source code to the public.
Simply put, the AGPLv3 is effectively the GPLv3, but with an additional licensing term that ensures that users who interact over a network with modified versions of the program can receive the source code for that program.
You are not modifying KoboToolbox, nor, as far as I know, are you offering a hosted service (modified or otherwise) to the public. You’ve written a software library to help people access KoboToolbox; you can license it as you please without conflicting with the AGPLv3. For comparison, the Python Requests library is licensed under Apache 2.0, and it’s not a license violation for Requests to connect to HTTP endpoints provided by AGPLv3 software.
For whatever it’s worth, the “MIT” license is actually GPL-compatible according to GNU:
This is a lax, permissive non-copyleft free software license, compatible with the GNU GPL.
Some people call this license “the MIT License,” but that term is misleading, since MIT has used many licenses for software. It is also ambiguous, since the same people also call the X11 license “the MIT License,” failing to distinguish them. We recommend not using the term “MIT License.”
The difference between the X11 license and the Expat license is that the X11 license contains an extra paragraph about using the X Consortium’s name. It is not a big deal, but it is a real difference.
For substantial programs it is better to use the Apache 2.0 license since it blocks patent treachery.
Thanks for your contribution, and good luck!