bump version to 4.8.0 (#190)

* bump version to 4.8.0

* update benchmark results on i7 & rpi

* correct python command on i7

* update results on jetson nano

* update results on atlas 200 cpu

* update results on vim3 cpu, sunrise x3 cpu

* update results on vim3 npu

* update results on edge2

* update results on rv1126 and vision2

* update results on axp

* update table

demo.py

@@ -6,7 +6,7 @@ import cv2 as cv
 from mp_palmdet import MPPalmDet
 
 # Check OpenCV version
-assert cv.__version__ >= "4.7.0", \
+assert cv.__version__ >= "4.8.0", \
        "Please install latest opencv-python to try this demo: python3 -m pip install --upgrade opencv-python"
 
 # Valid combinations of backends and targets