gh-100268: Add is_integer method to int (#100439)

This improves the lives of type annotation users of `float` - which type checkers implicitly treat as `int|float` because that is what most code actually wants. Before this change a `.is_integer()` method could not be assumed to exist on things annotated as `: float` due to the method not existing on both types.

Objects/clinic/longobject.c.h

@@ -467,4 +467,22 @@ skip_optional_kwonly:
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=bf6074ecf2f32cf4 input=a9049054013a1b77]*/
+
+PyDoc_STRVAR(int_is_integer__doc__,
+"is_integer($self, /)\n"
+"--\n"
+"\n"
+"Returns True. Exists for duck type compatibility with float.is_integer.");
+
+#define INT_IS_INTEGER_METHODDEF    \
+    {"is_integer", (PyCFunction)int_is_integer, METH_NOARGS, int_is_integer__doc__},
+
+static PyObject *
+int_is_integer_impl(PyObject *self);
+
+static PyObject *
+int_is_integer(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    return int_is_integer_impl(self);
+}
+/*[clinic end generated code: output=e518fe2b5d519322 input=a9049054013a1b77]*/

Objects/longobject.c

@@ -6168,6 +6168,19 @@ long_long_meth(PyObject *self, PyObject *Py_UNUSED(ignored))
     return long_long(self);
 }
 
+/*[clinic input]
+int.is_integer
+
+Returns True. Exists for duck type compatibility with float.is_integer.
+[clinic start generated code]*/
+
+static PyObject *
+int_is_integer_impl(PyObject *self)
+/*[clinic end generated code: output=90f8e794ce5430ef input=7e41c4d4416e05f2]*/
+{
+    Py_RETURN_TRUE;
+}
+
 static PyMethodDef long_methods[] = {
     {"conjugate",       long_long_meth, METH_NOARGS,
      "Returns self, the complex conjugate of any int."},
@@ -6186,6 +6199,7 @@ static PyMethodDef long_methods[] = {
     INT___GETNEWARGS___METHODDEF
     INT___FORMAT___METHODDEF
     INT___SIZEOF___METHODDEF
+    INT_IS_INTEGER_METHODDEF
     {NULL,              NULL}           /* sentinel */
 };