Make warnings accept a callable for showwarnings instead of
restricting itself to just functions and methods (which allows
built-in functions to be used, etc.).

Closes issue #10271. Thanks to lekma for the bug report.
diff --git a/Python/_warnings.c b/Python/_warnings.c
index 615a2d3..f8a7175 100644
--- a/Python/_warnings.c
+++ b/Python/_warnings.c
@@ -409,10 +409,10 @@
         else {
             PyObject *res;
 
-            if (!PyMethod_Check(show_fxn) && !PyFunction_Check(show_fxn)) {
+            if (!PyCallable_Check(show_fxn)) {
                 PyErr_SetString(PyExc_TypeError,
                                 "warnings.showwarning() must be set to a "
-                                "function or method");
+                                "callable");
                 Py_DECREF(show_fxn);
                 goto cleanup;
             }