CMake: Add python version requirement note to pro2cmake

Change-Id: Iccaf61e19fb7257ed21f656c287976d7d778ed1c
Reviewed-by: Leander Beernaert <leander.beernaert@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

util/cmake/pro2cmake.py

@@ -28,6 +28,9 @@
 #############################################################################
 
 
+# Requires Python 3.7. The import statement needs to be the first line of code
+# so it's not possible to conditionally check the version and raise an
+# exception.
 from __future__ import annotations
 
 import copy