Issue #27348: Restore “Exception: None” formatting in traceback module

This fixes a regression caused by revision 73afda5a4e4c. Also reverts the
decimal test workaround added in revision 5f3dd0a2b1ab.

Remove test_without_exception(). According to revision ecaafc32c500, this was
added in Python 2 so that print_exc() would output “None” when called with no
exception set. However print_exc() never worked like this in Python 3, and
the use case is not documented.

Restore TracebackCases class name (instead of SyntaxTracebackCases), because
the class also tests other exceptions.