[WIP] Add Support for Functions Returning datetime Objects

[MarkRoddy]

Note this is not presently working. There is an issue causing a segfault which is proving difficult to track down as it is occurring in a segment of code not affected by this change, implying it's a memory corruption issue say, rather than something easier to track down like a null pointer passed into a CPython function. Stacktrace below for reference:

    #0 0x7f1ac360e95f  (/lib/x86_64-linux-gnu/libpython3.8.so.1.0+0x25a95f)
    #1 0x7f1ac35e780a in PyUnicode_New (/lib/x86_64-linux-gnu/libpython3.8.so.1.0+0x23380a)
    #2 0x7f1ac35bb7f0 in _PyUnicodeWriter_PrepareInternal (/lib/x86_64-linux-gnu/libpython3.8.so.1.0+0x2077f0)
    #3 0x7f1ac35e02df in PyUnicode_DecodeUTF8Stateful (/lib/x86_64-linux-gnu/libpython3.8.so.1.0+0x22c2df)
    #4 0x7f1ac35e682c in PyUnicode_FromFormatV (/lib/x86_64-linux-gnu/libpython3.8.so.1.0+0x23282c)
    #5 0x7f1ac3567afb in PyErr_Format (/lib/x86_64-linux-gnu/libpython3.8.so.1.0+0x1b3afb)
    #6 0x7f1ac36126a2 in _PyObject_GenericGetAttrWithDict (/lib/x86_64-linux-gnu/libpython3.8.so.1.0+0x25e6a2)
    #7 0x7f1ac36130c9 in _PyModuleSpec_IsInitializing (/lib/x86_64-linux-gnu/libpython3.8.so.1.0+0x25f0c9)
    #8 0x7f1ac343ca78 in PyImport_ImportModuleLevelObject (/lib/x86_64-linux-gnu/libpython3.8.so.1.0+0x88a78)
    #9 0x7f1ac3559b0c in PyImport_ImportModuleLevel (/lib/x86_64-linux-gnu/libpython3.8.so.1.0+0x1a5b0c)
    #10 0x7f1ac3559cf4 in PyImport_Import (/lib/x86_64-linux-gnu/libpython3.8.so.1.0+0x1a5cf4)
    #11 0x7f1ac3559dfd in PyImport_ImportModule (/lib/x86_64-linux-gnu/libpython3.8.so.1.0+0x1a5dfd)
    #12 0x7f1ad8c6c5ab in pyudf::PythonFunction::init(std::__cxx11::basic_string<char, std::char_traits<char>, std::a\
llocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) /hom\
e/mark/development/duckdb-python-udf/src/python_function.cpp:28
    #13 0x7f1ad8c6c03b in pyudf::PythonFunction::PythonFunction(std::__cxx11::basic_string<char, std::char_traits<cha\
r>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > co\
nst&) /home/mark/development/duckdb-python-udf/src/python_function.cpp:19
    #14 0x7f1ad8c6f683 in pyudf::PythonTableFunction::PythonTableFunction(std::__cxx11::basic_string<char, std::char_\
traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<\
char> > const&) /home/mark/development/duckdb-python-udf/src/python_table_function.cpp:22