bpo-38237: Let pow() support keyword arguments (GH-16302) (GH-16320)

Backported with release manager approval

Python/bltinmodule.c

@@ -1797,22 +1797,22 @@ builtin_ord(PyObject *module, PyObject *c)
 /*[clinic input]
 pow as builtin_pow
 
-    x: object
-    y: object
-    z: object = None
-    /
+    base: object
+    exp: object
+    mod: object = None
 
-Equivalent to x**y (with two arguments) or x**y % z (with three arguments)
+Equivalent to base**exp (with two arguments) or base**exp % mod (with three arguments)
 
 Some types, such as ints, are able to use a more efficient algorithm when
 invoked using the three argument form.
 [clinic start generated code]*/
 
 static PyObject *
-builtin_pow_impl(PyObject *module, PyObject *x, PyObject *y, PyObject *z)
-/*[clinic end generated code: output=50a14d5d130d404b input=653d57d38d41fc07]*/
+builtin_pow_impl(PyObject *module, PyObject *base, PyObject *exp,
+                 PyObject *mod)
+/*[clinic end generated code: output=3ca1538221bbf15f input=bd72d0a0ec8e5eb5]*/
 {
-    return PyNumber_Power(x, y, z);
+    return PyNumber_Power(base, exp, mod);
 }