lv_conf_checker.py: change requirement of python 3.6 to >=3.6
lv_conf_checker.py: change requirement of python 3.6 to >=3.6

Many current OSes ship newer versions (e.g. 3.8) and they work OK
for this, so I don't think 3.6 is required specifically.
Versions older than 3.6 wouldn't work, though, since f-strings
are used.

littlevgl/lvgl,littlevgl/lvgl,littlevgl/lvgl,littlevgl/lvgl