readme.md: Adjust minimum required Python to 3.7 [skip ci] (#8060)

CMake checks for Python 3.7, see
https://github.com/danmar/cppcheck/blob/b66f900e2067006ca81eaa7648bdae9a5a7d6466/cmake/findDependencies.cmake#L58

Author: gruenich

readme.md

@@ -46,7 +46,7 @@ There are multiple compilation choices:
 * (Windows) Qt Creator + MinGW
 * GNU compilers - via make or directly
 
-The minimum required Python version is 3.6.
+The minimum required Python version is 3.7.
 
 ### CMake