You are distributing GPLv3-covered software. You are only able to do so under the conditions of the license, which include giving every recipient access to the complete corresponding source code of the software.
It is OK if this software includes components that are not licensed under the GPL. In that case, one of the following scenarios must apply:
the other license is compatible with the GPL, so that the software as whole (including the non-GPL components) can be distributed under the terms of the GPL. For the GPLv3 a number of licenses are suitable here, such as MIT, BSD, Apache-2, or LGPLv3.
Compare also the GPL compatibility matrix, although it only covers GPL-family licenses.
The roles are a bit reversed here, e.g. your file would be library like and shall be used by the GPLv3 software.
the other component falls under the system libraries exception, for examples components that are part of the operating system
the other component is a “generally available free [program] which [is] used unmodified [but is] not part of the work”
the copyright holders have issued an exception to these requirements so that the GPLv3 software can be combined with differently-licensed components as well. The LGPLv3 or the Classpath Exception are examples of such an exception.
You want to modify/extend the software, but want to license your extension in a way that doesn't require source disclosure.
Considering the above scenarios, this is not possible:
- You still have to provide the complete corresponding source code for the software. This would include your extensions.
- You must license your extensions under a GPL-compatible license.
- None of the exceptions would apply here. Your modifications are not a system library, your modifications are a generally available free program, and you have not received a special exception from the copyright holders.
You do not have to publicly offer your source code. However, you must provide the source code to any recipients of the software. This is easiest to do if you always provide an archive of the source code alongside the installed software. As you said, your users are non-technical and are therefore unlikely to share the source code further, even though they would have the right to.
If you were to distribute the software with your modifications, without licensing the complete corresponding source code of the software under a GPL-compatible license, that would be a license violation, and your license to the software automatically terminates. Any further sharing of the software or of your modifications or making further modifications would then be copyright infringement.
The installed form of a Python software is not generally identical with its source code in the sense of the GPL. For the GPL, the source code is the preferred form of making modifications, and includes build scripts such as setup.py. It could also be argued that test files are part of the source code.