run_unittests: Disable pytest with python <= 3.5
run_unittests: Disable pytest with python <= 3.5

It's causing issues in some CI runners, it looks like it could be that
bug: https://github.com/pytest-dev/pytest-xdist/issues/204.

pexip/meson,jpakkane/meson,becm/meson,mesonbuild/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,jpakkane/meson,becm/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,becm/meson,QuLogic/meson,jpakkane/meson,pexip/meson,QuLogic/meson,jpakkane/meson,becm/meson,jpakkane/meson,becm/meson,pexip/meson,mesonbuild/meson,QuLogic/meson,becm/meson,pexip/meson,pexip/meson,becm/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,QuLogic/meson,becm/meson,becm/meson,mesonbuild/meson,mesonbuild/meson,QuLogic/meson,becm/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson