gh-129005: Update _pyio.BytesIO to use bytearray.resize on write (#129702)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: cmaloney

Lib/_pyio.py

@@ -937,10 +937,8 @@ def write(self, b):
             return 0
         pos = self._pos
         if pos > len(self._buffer):
-            # Inserts null bytes between the current end of the file
-            # and the new write position.
-            padding = b'\x00' * (pos - len(self._buffer))
-            self._buffer += padding
+            # Pad buffer to pos with null bytes.
+            self._buffer.resize(pos)
         self._buffer[pos:pos + n] = b
         self._pos += n
         return n