Adapt array module declarations Python 3.15 changes

In python/cpython#148676 the format
of the `arraydescr` struct get changed, which meant that our
representation was outdated and crashed.

It isn't easy to adapt it perfectly for both (if users are
manually using the `arraydescr` type anyway) but we can make
our `array.array.__getbuffer__` function not crash by hiding
some of the details in C code.

Author: da-woods

Cython/Includes/cpython/array.pxd

@@ -70,11 +70,14 @@ cdef extern from *:  # Hard-coded utility code hack.
     ctypedef object GETF(array a, Py_ssize_t ix)
     ctypedef object SETF(array a, Py_ssize_t ix, object o)
     ctypedef struct arraydescr:  # [object arraydescr]:
-        char typecode
+        char typecode   # This is untrue in Python 3.15+ but it isn't easy to expose both
         int itemsize
         GETF getitem    # PyObject * (*getitem)(struct arrayobject *, Py_ssize_t);
         SETF setitem    # int (*setitem)(struct arrayobject *, Py_ssize_t, PyObject *);
 
+    Py_ssize_t _typecode_length "__Pyx_PyArrayDescr_typecode_length"(arraydescr *descr)
+    void _copy_typecode "__Pyx_PyArrayDescr_copy_typecode"(arraydescr *descr, char *dst)
+
     ctypedef union __data_union "__Pyx_data_union":
         # views of ob_item:
         float* as_floats        # direct float pointer access to buffer
@@ -118,15 +121,14 @@ cdef extern from *:  # Hard-coded utility code hack.
             info.itemsize = self.ob_descr.itemsize   # e.g. sizeof(float)
             info.len = info.itemsize * item_count
 
-            info.shape = <Py_ssize_t*> PyObject_Malloc(sizeof(Py_ssize_t) + 2)
+            info.shape = <Py_ssize_t*> PyObject_Malloc(sizeof(Py_ssize_t) + _typecode_length(self.ob_descr) +  1)
             if not info.shape:
                 raise MemoryError()
             info.shape[0] = item_count      # constant regardless of resizing
             info.strides = &info.itemsize
 
             info.format = <char*> (info.shape + 1)
-            info.format[0] = self.ob_descr.typecode
-            info.format[1] = 0
+            _copy_typecode(self.ob_descr, info.format)
             info.obj = self
 
         def __releasebuffer__(self, Py_buffer* info):

Cython/Utility/arrayarray.h

@@ -23,13 +23,36 @@
 // below.  That's defined later because the appropriate get and set
 // functions aren't visible yet.
 typedef struct arraydescr {
-    int typecode;
+#if PY_VERSION_HEX <= 0x030F00a8
+    const char* typecode;
+#else
+    char typecode;
+#endif
     int itemsize;
     PyObject * (*getitem)(struct arrayobject *, Py_ssize_t);
     int (*setitem)(struct arrayobject *, Py_ssize_t, PyObject *);
+#if PY_VERSION_HEX <= 0x030F00a8
     char *formats;
+#endif
 } arraydescr;
 
+static CYTHON_INLINE Py_ssize_t __Pyx_PyArrayDescr_typecode_length(arraydescr *desc) {
+#if PY_VERSION_HEX <= 0x030F00a8
+    return strlen(desc->typecode);
+#else
+    return 1;
+#endif
+}
+
+static CYTHON_INLINE void __Pyx_PyArrayDescr_copy_typecode(arraydescr *desc, char *dst) {
+#if PY_VERSION_HEX <= 0x030F00a8
+    strcpy(dst, desc->typecode);
+#else
+    dst[0] = arrayarray->typecode;
+    dst[1] = '\0';
+#endif
+}
+
 typedef union {
     char *ob_item;
     float *as_floats;