[BUG] py312 compatibility

[tacaswell]

Describe the bug
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python/cpython#92537 removed the function _PyUnicode_Ready which cython is using.

To Reproduce
Code to reproduce the behaviour:

Attempt to build scipy fails with missing symbols. It looks like this changes was anticipated, however the code is guarded on defined(PyUnicode_IS_READY) and CPyhon retained that function as a no-op for back-compatibility. Checking if the private function is defined fixes this problem (but exposes another that I will open an issue for shortly).

I will open a PR shortly with the diff

$ git diff
diff --git a/Cython/Utility/ModuleSetupCode.c b/Cython/Utility/ModuleSetupCode.c
index f05840161..10b7b887d 100644
--- a/Cython/Utility/ModuleSetupCode.c
+++ b/Cython/Utility/ModuleSetupCode.c
@@ -928,7 +928,7 @@ static CYTHON_INLINE PyObject * __Pyx_PyDict_GetItemStrWithError(PyObject *dict,
   /* new Py3.3 unicode type (PEP 393) */
   #define CYTHON_PEP393_ENABLED 1
 
-  #if defined(PyUnicode_IS_READY)
+  #if defined(_PyUnicode_Ready)
   #define __Pyx_PyUnicode_READY(op)       (likely(PyUnicode_IS_READY(op)) ? \
                                               0 : _PyUnicode_Ready((PyObject *)(op)))
   #else
@@ -943,7 +943,7 @@ static CYTHON_INLINE PyObject * __Pyx_PyDict_GetItemStrWithError(PyObject *dict,
   #define __Pyx_PyUnicode_DATA(u)         PyUnicode_DATA(u)
   #define __Pyx_PyUnicode_READ(k, d, i)   PyUnicode_READ(k, d, i)
   #define __Pyx_PyUnicode_WRITE(k, d, i, ch)  PyUnicode_WRITE(k, d, i, ch)
-  #if defined(PyUnicode_IS_READY) && defined(PyUnicode_GET_SIZE)
+  #if defined(_PyUnicode_Ready) && defined(PyUnicode_GET_SIZE)
   #if CYTHON_COMPILING_IN_CPYTHON && PY_VERSION_HEX >= 0x03090000
   // Avoid calling deprecated C-API functions in Py3.9+ that PEP-623 schedules for removal in Py3.12.
   // https://www.python.org/dev/peps/pep-0623/

Expected behavior
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Environment (please complete the following information):

• OS: [e.g. Linux, Windows, macOS] Linux
• Python version [e.g. 3.8.4] current main (py312)
• Cython version [e.g. 0.29.18] current master

Additional context
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[da-woods]

Closed by b4595a0 I think?

[tacaswell]

I can confirm that this is fixed.