Comply with CMP0148

See https://cmake.org/cmake/help/latest/policy/CMP0148.html

CMakeLists.txt

@@ -107,14 +107,7 @@ if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
   set( CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX} )
 
   if(LLVM_INCLUDE_TESTS)
-    set(Python_ADDITIONAL_VERSIONS 2.7)
-    include(FindPythonInterp)
-    if(NOT PYTHONINTERP_FOUND)
-      message(FATAL_ERROR
-"Unable to find Python interpreter, required for builds and testing.
-
-Please install Python or specify the Python3_EXECUTABLE CMake variable.")
-    endif()
+    find_package(Python3 3.8 REQUIRED COMPONENTS Interpreter)
 
     # Check prebuilt llvm/utils.
     if(EXISTS ${LLVM_TOOLS_BINARY_DIR}/FileCheck${CMAKE_EXECUTABLE_SUFFIX}