pythongh-93575: Use correct way to calculate PyUnicode struct sizes

Author: tiran

Lib/test/test_unicode.py

@@ -2370,9 +2370,10 @@ def test_expandtabs_optimization(self):
         self.assertIs(s.expandtabs(), s)
 
     def test_raiseMemError(self):
-        null_byte = 1
-        ascii_struct_size = sys.getsizeof("a") - len("a") - null_byte
-        compact_struct_size = sys.getsizeof("\xff") - len("\xff") - null_byte
+        asciifields = "nnb"
+        compactfields = asciifields + "nP"
+        ascii_struct_size = support.calcobjsize(asciifields)
+        compact_struct_size = support.calcobjsize(compactfields)
 
         for char in ('a', '\xe9', '\u20ac', '\U0010ffff'):
             code = ord(char)

Misc/NEWS.d/next/Tests/2022-06-08-14-17-59.gh-issue-93575.Xb2LNB.rst

@@ -0,0 +1,4 @@
+Fix issue with test_unicode test_raiseMemError. The test case now use
+``test.support.calcobjsize`` to calculate size of PyUnicode structs.
+:func:`sys.getsizeof` may return different size when string has UTF-8
+memory.