apacheGH-40153: [Python] Make Tensor.__getbuffer__ work on 32-bit p…

…latforms (apache#40294)

### Rationale for this change

`Tensor.__getbuffer__` would silently assume that `Py_ssize_t` is the same width as `int64_t`, which is true only on 64-bit platforms.

### What changes are included in this PR?

Create an internal buffer of `Py_ssize_t` values mirroring a Tensor's shape and strides, to avoid relying on the aforementioned assumption.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.

* GitHub Issue: apache#40153

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>

python/pyarrow/lib.pxd

@@ -295,6 +295,8 @@ cdef class Tensor(_Weakrefable):
 
     cdef readonly:
         DataType type
+        bytes _ssize_t_shape
+        bytes _ssize_t_strides
 
     cdef void init(self, const shared_ptr[CTensor]& sp_tensor)
 

python/pyarrow/tensor.pxi

@@ -15,6 +15,9 @@
 # specific language governing permissions and limitations
 # under the License.
 
+# Avoid name clash with `pa.struct` function
+import struct as _struct
+
 
 cdef class Tensor(_Weakrefable):
     """
@@ -40,6 +43,14 @@ cdef class Tensor(_Weakrefable):
         self.sp_tensor = sp_tensor
         self.tp = sp_tensor.get()
         self.type = pyarrow_wrap_data_type(self.tp.type())
+        self._ssize_t_shape = self._make_shape_or_strides_buffer(self.shape)
+        self._ssize_t_strides = self._make_shape_or_strides_buffer(self.strides)
+
+    def _make_shape_or_strides_buffer(self, values):
+        """
+        Make a bytes object holding an array of `values` cast to `Py_ssize_t`.
+        """
+        return _struct.pack(f"{len(values)}n", *values)
 
     def __repr__(self):
         return """<pyarrow.Tensor>
@@ -282,10 +293,8 @@ strides: {0.strides}""".format(self)
             buffer.readonly = 0
         else:
             buffer.readonly = 1
-        # NOTE: This assumes Py_ssize_t == int64_t, and that the shape
-        # and strides arrays lifetime is tied to the tensor's
-        buffer.shape = <Py_ssize_t *> &self.tp.shape()[0]
-        buffer.strides = <Py_ssize_t *> &self.tp.strides()[0]
+        buffer.shape = <Py_ssize_t *> cp.PyBytes_AsString(self._ssize_t_shape)
+        buffer.strides = <Py_ssize_t *> cp.PyBytes_AsString(self._ssize_t_strides)
         buffer.suboffsets = NULL