cmake: Fixed a bug where python 3.8 was not detected on Windows

Fixed a bug where python 3.8 was not detected on Windows.

Python 3.8 was released two weeks ago and any Windows user that
installs it will detect the wrong version.

Also add detection for 3.9 for future-proofing.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
diff --git a/cmake/backports/FindPythonInterp.cmake b/cmake/backports/FindPythonInterp.cmake
index 634c185..20eb7ae 100644
--- a/cmake/backports/FindPythonInterp.cmake
+++ b/cmake/backports/FindPythonInterp.cmake
@@ -79,7 +79,7 @@
 
 set(_PYTHON1_VERSIONS 1.6 1.5)
 set(_PYTHON2_VERSIONS 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0)
-set(_PYTHON3_VERSIONS 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0)
+set(_PYTHON3_VERSIONS 3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0)
 
 if(PythonInterp_FIND_VERSION)
     if(PythonInterp_FIND_VERSION_COUNT GREATER 1)