bpo-37467: Fix PyErr_Display() for bytes filename (GH-14504)

Fix sys.excepthook() and PyErr_Display() if a filename is a bytes
string. For example, for a SyntaxError exception where the filename
attribute is a bytes string.

Cleanup also test_sys:

* Sort imports.
* Rename numruns global var to INTERN_NUMRUNS.
* Add DisplayHookTest and ExceptHookTest test case classes.
* Don't save/restore sys.stdout and sys.displayhook using
  setUp()/tearDown(): do it in each test method.
* Test error case (call hook with no argument) after the success case.

Python/pythonrun.c

@@ -797,7 +797,7 @@ print_exception(PyObject *f, PyObject *value)
             Py_DECREF(value);
             value = message;
 
-            line = PyUnicode_FromFormat("  File \"%U\", line %d\n",
+            line = PyUnicode_FromFormat("  File \"%S\", line %d\n",
                                           filename, lineno);
             Py_DECREF(filename);
             if (line != NULL) {