Fix XFAIL import in test failure workaround.

sympy.spec

@@ -94,7 +94,7 @@ cp -a . %{py3dir}
 # One plotting test expects one warning, which happens with python < 3.4, but
 # the same warning is triggered in a different spot with python 3.4, leading
 # to an assertion failure.
-sed -e 's/import warnings/&, XFAIL/' \
+sed -e '/import warnings/ifrom sympy.utilities.pytest import XFAIL' \
     -e '/^def test_matplotlib():/i@XFAIL' \
     -i %{py3dir}/sympy/plotting/tests/test_plot_implicit.py