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.
(cherry picked from commit f9b7457bd7f438263e0d2dd1f70589ad56a2585e)

Co-authored-by: Victor Stinner <vstinner@redhat.com>

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