package/python-mwclient: require pytest-runner only when necessary

package/python-mwclient: require pytest-runner only when necessary

Fixes
http://autobuild.buildroot.net/results/09c/09c0e590448f1fc069d1d00564c202d2b009d59a/
http://autobuild.buildroot.net/results/79e/79ef04d2e8c91231028e23f4bb35df685efa55ca/
http://autobuild.buildroot.net/results/7a4/7a4a4aecbe6dd315062bafe97b291645d3d6ced1/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>