revlog: use appropriate format char for int ("i" instead of "I")

From https://docs.python.org/3.8/c-api/arg.html#numbers :

i (int) [int]
Convert a Python integer to a plain C int.

I (int) [unsigned int]
Convert a Python integer to a C unsigned int, without overflow checking.

mercurial/cext/revlog.c

@@ -491,7 +491,7 b' static PyObject *index_pack_header(index'
 {
 	int header;
 	char out[4];
-	if (!PyArg_ParseTuple(args, "I", &header)) {
+	if (!PyArg_ParseTuple(args, "i", &header)) {
 		return NULL;
 	}
 	if (self->format_version != format_v1) {