Avoid using PyUnicode_GET_SIZE() in PEP393 code.

Cython/Utility/FunctionArguments.c

@@ -239,7 +239,7 @@ static int __Pyx_ParseOptionalKeywords(
             while (*name) {
                 int cmp = (**name == key) ? 0 :
                 #if !CYTHON_COMPILING_IN_PYPY && PY_MAJOR_VERSION >= 3
-                    (PyUnicode_GET_SIZE(**name) != PyUnicode_GET_SIZE(key)) ? 1 :
+                    (__Pyx_PyUnicode_GET_LENGTH(**name) != __Pyx_PyUnicode_GET_LENGTH(key)) ? 1 :
                 #endif
                     // need to convert argument name from bytes to unicode for comparison
                     PyUnicode_Compare(**name, key);
@@ -257,7 +257,7 @@ static int __Pyx_ParseOptionalKeywords(
                 while (argname != first_kw_arg) {
                     int cmp = (**argname == key) ? 0 :
                     #if !CYTHON_COMPILING_IN_PYPY && PY_MAJOR_VERSION >= 3
-                        (PyUnicode_GET_SIZE(**argname) != PyUnicode_GET_SIZE(key)) ? 1 :
+                        (__Pyx_PyUnicode_GET_LENGTH(**argname) != __Pyx_PyUnicode_GET_LENGTH(key)) ? 1 :
                     #endif
                         // need to convert argument name from bytes to unicode for comparison
                         PyUnicode_Compare(**argname, key);