*: Fix pyflakes

    (zdev+py39.env) kirr@deca:~/src/wendelin/z/zodbtools/zodbtools$ python3 -m pyflakes .
    ./test/test_dump.py:32:1: 'pytest.xfail' imported but unused

Unused xfail import was there in test_dump.py from 425e6656 (dump,
commit: Test for both non-empty and empty transaction extensions).

zodbtools/test/test_dump.py

@@ -29,7 +29,7 @@ from ZODB.utils import p64
 from io import BytesIO
 
 from zodbtools.test.testutil import fs1_testdata_py23
-from pytest import mark, raises, xfail
+from pytest import mark, raises
 
 from six import PY3