Fix misuse of PyUnicode_GET_SIZE() => PyUnicode_GET_LENGTH()

And PyUnicode_GetSize() => PyUnicode_GetLength()
diff --git a/Modules/_csv.c b/Modules/_csv.c
index c02ee38..443309c 100644
--- a/Modules/_csv.c
+++ b/Modules/_csv.c
@@ -207,7 +207,7 @@
         *target = '\0';
         if (src != Py_None) {
             Py_ssize_t len;
-            len = PyUnicode_GetSize(src);
+            len = PyUnicode_GetLength(src);
             if (len > 1) {
                 PyErr_Format(PyExc_TypeError,
                     "\"%s\" must be an 1-character string",