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Some geometry extensions do not provide a PyObject as they do not have a Python counterpart
as it would serve no purpose to have it.
This commit handles this situation making sure to provide the right error to Python
or to ignore the extension where appropriate.
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Minor refactoring to centralise string to enum conversion in a single place
When using gcc 10 and PyCXX 7.1.4 on Fedora there is an error because the conversion from size_t to Py_ssize_t is ambiguous
See https://forum.freecadweb.org/viewtopic.php?f=4&t=52502
The class Py::Tuple is also incorrectly used here and causes a memory leak
