tests/qapi-schema: Use Python OSError instead of outmoded IOError

tests/qapi-schema: Use Python OSError instead of outmoded IOError

https://docs.python.org/3.6/library/exceptions.html has

    Changed in version 3.3: EnvironmentError, IOError, WindowsError,
    socket.error, select.error and mmap.error have been merged into
    OSError, and the constructor may return a subclass.

and

    The following exceptions are kept for compatibility with previous
    versions; starting from Python 3.3, they are aliases of OSError.

    exception EnvironmentError

    exception IOError

    exception WindowsError

        Only available on Windows.

Switch to the preferred name.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210922125619.670673-2-armbru@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[Details added to commit message]