package/python-flup: update dependency to python3

package/python-flup: update dependency to python3

Since version flup-1.0.3.dev20151210, flup needs Python 3. This was
apparently missed in Buildroot commit
ff0f53c04db8cc6eb5ce2eb92b7e7d1fe17297ae, which bumped flup from
1.0.3.dev-20110405 to 1.0.3.dev20161029.

Signed-off-by: Jared Bents <jared.bents@rockwellcollins.com>
Signed-off-by: Ryan Barnett <ryan.barnett@rockwellcollins.com>
Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>