Document that Python 3 is not supported

Task-number: QTBUG-60163
Change-Id: I4763dca3c651c08fb4094a3def6494b3dcd65ebd
Reviewed-by: Florian Bruhin <qt-project.org@the-compiler.org>
Reviewed-by: Michal Klocek <michal.klocek@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/webengine/doc/src/qtwebengine-platform-notes.qdoc

@@ -62,7 +62,7 @@
     On all platforms, the following tools are required:
 
     \list
-        \li \l Python 2.7.5 or later
+        \li \l Python 2.7.5 or later. Python 3 is not supported.
         \li Bison, Flex
         \li GPerf
     \endlist