CLN: _libs.interval looping with cdef index (#32329)

* CLN: using "n" variable to loop

* cdef for "i" as index and for "n" as the length

Author: ShaharNaveh

pandas/_libs/interval.pyx

@@ -501,14 +501,14 @@ def intervals_to_interval_bounds(ndarray intervals, bint validate_closed=True):
     """
     cdef:
         object closed = None, interval
-        int64_t n = len(intervals)
+        Py_ssize_t i, n = len(intervals)
         ndarray left, right
         bint seen_closed = False
 
     left = np.empty(n, dtype=intervals.dtype)
     right = np.empty(n, dtype=intervals.dtype)
 
-    for i in range(len(intervals)):
+    for i in range(n):
         interval = intervals[i]
         if interval is None or util.is_nan(interval):
             left[i] = np.nan