README: Change recommended python version
Recommend  3.8 and 3.9 from 3.7 because of frequently tested.
@@ -65,7 +65,7 @@ Requirements
------------
- Minimum Python version: 3.6
-- Recommended Python version: 3.7.5 or later
+- Recommended Python version: 3.8, 3.9 (frequently tested on)
- Dependent libraries: requests, py7zr
