cext: cast s# arguments of Py_BuildValue() to Py_ssize_t

cext: cast s# arguments of Py_BuildValue() to Py_ssize_t

The doc doesn't state that "s#" of Py_BuildValue() is controlled by
PY_SSIZE_T_CLEAN (unlike the one for PyArg_ParseTuple()), but actually
it's switched to Py_ssize_t.

https://docs.python.org/2/c-api/arg.html#c.Py_BuildValue
https://github.com/python/cpython/blob/2.7/Python/modsupport.c#L432

Follow up for b01bbb8ff1f2 and 896b19d12c08.

--- a/mercurial/cext/parsers.c	Mon Apr 08 10:52:04 2019 -0400
+++ b/mercurial/cext/parsers.c	Tue Apr 09 21:59:37 2019 +0900
@@ -184,7 +184,8 @@
 		goto quit;
 	}
 
-	parents = Py_BuildValue(PY23("s#s#", "y#y#"), str, 20, str + 20, 20);
+	parents = Py_BuildValue(PY23("s#s#", "y#y#"), str, (Py_ssize_t)20,
+	                        str + 20, (Py_ssize_t)20);
 	if (!parents) {
 		goto quit;
 	}

--- a/mercurial/cext/revlog.c	Mon Apr 08 10:52:04 2019 -0400
+++ b/mercurial/cext/revlog.c	Tue Apr 09 21:59:37 2019 +0900
@@ -366,7 +366,7 @@
 
 	entry = Py_BuildValue(tuple_format, offset_flags, comp_len, uncomp_len,
 	                      base_rev, link_rev, parent_1, parent_2, c_node_id,
-	                      20);
+	                      (Py_ssize_t)20);
 
 	if (entry) {
 		PyObject_GC_UnTrack(entry);
@@ -3017,8 +3017,9 @@
 	PyModule_AddObject(mod, "nodetree", (PyObject *)&nodetreeType);
 
 	if (!nullentry) {
-		nullentry = Py_BuildValue(PY23("iiiiiiis#", "iiiiiiiy#"), 0, 0,
-		                          0, -1, -1, -1, -1, nullid, 20);
+		nullentry =
+		    Py_BuildValue(PY23("iiiiiiis#", "iiiiiiiy#"), 0, 0, 0, -1,
+		                  -1, -1, -1, nullid, (Py_ssize_t)20);
 	}
 	if (nullentry)
 		PyObject_GC_UnTrack(nullentry);