I'd like to mangle and demangle C++ function names in a Python program.
Is there anything like that available? I searched for hours now, perhaps I'm lucky here...
You most likely don't want to be doing this in Python. As an aside you probably shouldn't be exporting mangled names from your DLLs since it makes it hard to use for anyone with a different compiler.
If you have to use mangled names then just hard code them in your Python code. If you were going to do mangling in Python code then you'd have to:

Know the implementation specific rules for the compiler in question.
Specify in Python the C++ function signature for each function.

It seems highly unlikely to me that coding all this up in Python would be better than simply hard coding the mangled names.
C++ Name Mangling Library for Python