Issue #13093: Fix _testcapi.unicode_encodedecimal()

_testcapimodule.c is not "ssize_t" safe in Python 2.7: the length argument type
is int, not Py_ssize_t.

Modules/_testcapimodule.c

@@ -1109,7 +1109,7 @@ static PyObject *
 unicode_encodedecimal(PyObject *self, PyObject *args)
 {
     Py_UNICODE *unicode;
-    Py_ssize_t length;
+    int length;
     char *errors = NULL;
     PyObject *decimal;
     Py_ssize_t decimal_length, new_length;