More efficient (for finite x) handling of special cases in math.modf

Rewrite changes in python#102523

Author: skirpichev

Modules/mathmodule.c

@@ -2190,10 +2190,12 @@ math_modf_impl(PyObject *module, double x)
     double y;
     /* some platforms don't do the right thing for NaNs and
        infinities, so we take care of special cases directly. */
-    if (Py_IS_INFINITY(x))
-        return Py_BuildValue("(dd)", copysign(0., x), x);
-    else if (Py_IS_NAN(x))
-        return Py_BuildValue("(dd)", x, x);
+    if (!Py_IS_FINITE(x)) {
+        if (Py_IS_INFINITY(x))
+            return Py_BuildValue("(dd)", copysign(0., x), x);
+        else
+            return Py_BuildValue("(dd)", x, x);
+    }
 
     errno = 0;
     x = modf(x, &y);