Changed build to require python3 instead of python2

CMakeLists.txt

@@ -432,11 +432,11 @@ else()
 endif()
 
 # Python is used for the manual and (possibly) message catalogs
-find_package( Python2 )
-if( Python2_FOUND )
-   set( PYTHON "${Python2_EXECUTABLE}" )
+find_package( Python3 )
+if( Python3_FOUND )
+   set( PYTHON "${Python3_EXECUTABLE}" )
 elseif( CMAKE_SYSTEM_NAME MATCHES "Windows" )
-   nuget_package( pkgdir "python2" "2.7.17" )
+   nuget_package( pkgdir "python3" "3.7.7" )
    file( TO_NATIVE_PATH "${pkgdir}/tools/python.exe" PYTHON )
 endif()