Upgrade Cython to version 3

[LasseBlaauwbroek]

While looking at #333, I hypothesized that upgrading Cython might solve the issue. It didn't. But upgrading should still happen at some point. This is my work in progress on that. The tests pass, but there are two main things missing:

Problem (1):
Starting with Cython 3, you can only do except+ or except +reraise_kj_exception on extern functions coming from C++. (This makes sense, and the way things were declared in Pycapnp wasn't too good.) As a result, I had to remove a lot of these declarations. This results in some segmentation faults, because Cython no longer detects C++ exceptions and converts them to Python exceptions in some places.

To solve this, all extern declarations in .pxd files have to be examined and except +reraise_kj_exception clauses need to be added to anything that might throw. Previously, this was done really inconsistently. The lazy solution would be to just add the clause everywhere, but I'm not sure what the performance implications are.

Problem (2):
The compilation output of python setup.py build_ext --inplace is now full of messages like these:

capnp/lib/capnp.cpp: In function ‘PyObject* __pyx_f_5capnp_3lib_5capnp_18_DynamicListReader__get(__pyx_obj_5capnp_3lib_5capnp__DynamicListReader*, int64_t, int)’:
capnp/lib/capnp.cpp:4871:51: warning: moving a temporary object prevents copy elision [-Wpessimizing-move]
 4871 |   #define __PYX_STD_MOVE_IF_SUPPORTED(x) std::move(x)
      |                                          ~~~~~~~~~^~~
capnp/lib/capnp.cpp:20944:59: note: in expansion of macro ‘__PYX_STD_MOVE_IF_SUPPORTED’
20944 |   __pyx_t_2 = __pyx_f_5capnp_3lib_5capnp_to_python_reader(__PYX_STD_MOVE_IF_SUPPORTED((( ::capnp::DynamicValue::Reader)__pyx_t_7)), __pyx_t_1); if (unlikely(!__pyx_t_2)) __PYX_ERR(0, 419, __pyx_L1_error)
      |                                                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~
capnp/lib/capnp.cpp:4871:51: note: remove ‘std::move’ call
 4871 |   #define __PYX_STD_MOVE_IF_SUPPORTED(x) std::move(x)
      |                                          ~~~~~~~~~^~~
capnp/lib/capnp.cpp:20944:59: note: in expansion of macro ‘__PYX_STD_MOVE_IF_SUPPORTED’
20944 |   __pyx_t_2 = __pyx_f_5capnp_3lib_5capnp_to_python_reader(__PYX_STD_MOVE_IF_SUPPORTED((( ::capnp::DynamicValue::Reader)__pyx_t_7)), __pyx_t_1); if (unlikely(!__pyx_t_2)) __PYX_ERR(0, 419, __pyx_L1_error)
      |                                                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~

There are to many move calls inserted. I'm not sure if this is a Cython issue, or if we are somehow annotating things wrong. Might be worth asking the Cython people.

I'm not planning on working on this further in the short term. If someone wants to take over on this, feel free.