FindCustomPython3: increase the minimum Python version to 3.8

cmake/multipython/FindCustomPython3.cmake

@@ -19,7 +19,7 @@ function(find_custom_python3)
     # the given Python version; otherwise, CMake will re-use the old variables.
     unset(Python3_DIR CACHE)
 
-    set(version_minor_min 7) # minimum Python version = 3.7
+    set(version_minor_min 8) # minimum Python version = 3.8
     set(Python3_FIND_UNVERSIONED_NAMES FIRST) # to cope with multiple installed Py minor versions
 
     # if DEFAULT_PATH is set, the Python platform will be sought in