Look for Python3 again in SearchInstalledSoftware.cmake

This commit essentially reverts a288ff9 from four years ago, which was
also part of the multi-python machinery that is not necessary anymore
after Python 2 support was dropped.

Also, make the choice of looked-up components consistent with what is
actually required.

CMakeLists.txt

@@ -113,11 +113,7 @@ if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
       message(FATAL_ERROR
 "Unable to find Python interpreter, required for builds and testing.
 
-Please install Python or specify the PYTHON_EXECUTABLE CMake variable.")
-    endif()
-
-    if( ${PYTHON_VERSION_STRING} VERSION_LESS 2.7 )
-      message(FATAL_ERROR "Python 2.7 or newer is required")
+Please install Python or specify the Python3_EXECUTABLE CMake variable.")
     endif()
 
     # Check prebuilt llvm/utils.