In C++ we do not support QgsProviderMetadata(key, description, createFunc) anymore,
but for python code we can have a patched class that still supports createFunc.
Why not keep the variant with createFunc in C++ as well? Because... SIP!
With newly added virtual methods in QgsProviderMetadata, SIP started to generate
a sip-specific subclass to handle derived classes in Python, however due to the variant
with PyObject* and custom MethodCode it was getting confused about what constructors
are available in C++ and failing to compile.
