disable some f-string formatting tests that give "suboptimal" results in Py2.6
disable some f-string formatting tests that give "suboptimal" results in Py2.6

ChristopherHogan/cython,c-blake/cython,ChristopherHogan/cython,da-woods/cython,fabianrost84/cython,scoder/cython,da-woods/cython,fabianrost84/cython,scoder/cython,cython/cython,cython/cython,scoder/cython,ChristopherHogan/cython,cython/cython,c-blake/cython,c-blake/cython,fabianrost84/cython,da-woods/cython,fabianrost84/cython,scoder/cython,cython/cython,c-blake/cython,c-blake/cython,fabianrost84/cython,da-woods/cython