python-treq: needs host-python-incremental

The python-treq package lists the incremental Python module as part of
its setup_requires variable in setup.py, so it must be added as a host
dependency of the python-treq package to avoid build failures.

So far, this issue wasn't visible because python-treq selects
python-twisted, which itself selects the target python-incremental
package. Because python-incremental was before python-treq in the
alphabetic ordering, it was always built before python-treq. And due
to the fact that PYTHONPATH currently contains the directory with
target Python modules, the host Python interpreter was happily using
the target python-incremental while running on the host. But as we are
going to clean up PYTHONPATH, this will no longer be the case, and
hence python-treq needs to be fixed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>