Remove C++/C99-style comments.

Python/_warnings.c

@@ -839,9 +839,9 @@ create_filter(PyObject *category, const char *action)
 static PyObject *
 init_filters(void)
 {
-    // Don't silence DeprecationWarning if -3 was used.
+    /* Don't silence DeprecationWarning if -3 was used. */
     PyObject *filters = PyList_New(Py_Py3kWarningFlag ? 3 : 4);
-    unsigned int pos = 0;  // Post-incremented in each use.
+    unsigned int pos = 0;  /* Post-incremented in each use. */
     unsigned int x;
     const char *bytes_action;