PEP 756: Fix buffer format (#3967)

Use the native encoding. "=I" size is 4 bytes on all platforms.

Author: vstinner

peps/pep-0756.rst

@@ -171,8 +171,8 @@ export format:
 Export format               Buffer format       Item size
 ==========================  ==================  ============
 ``PyUnicode_FORMAT_UCS1``   ``"B"``             1 byte
-``PyUnicode_FORMAT_UCS2``   ``"H"``             2 bytes
-``PyUnicode_FORMAT_UCS4``   ``"I"`` or ``"L"``  4 bytes
+``PyUnicode_FORMAT_UCS2``   ``"=H"``            2 bytes
+``PyUnicode_FORMAT_UCS4``   ``"=I"``            4 bytes
 ``PyUnicode_FORMAT_UTF8``   ``"B"``             1 byte
 ``PyUnicode_FORMAT_ASCII``  ``"B"``             1 byte
 ==========================  ==================  ============