[RFC][OpenMP] Update to Python3 for lit test

[tianshilei1992]

Summary

I think it's reasonable to upgrade to Python 3 for LIT test requirement because lit itself (llvm/utils/lit/lit.py) already switched to Python 3. In addition, LLVM already requires Python 3.6 to be the minimum version (https://llvm.org/docs/GettingStarted.html#software).

[tianshilei1992]

remove development as we only need interpreter

[tianshilei1992]

ping

[jhuber6]

Probably should've discussed this at Wednesday's OpenMP meeting, I definitely think we should upgrade.

[jdoerfert]

if llvm requires 3, let's do it.

openmp/cmake/OpenMPTesting.cmake

 # Keep track if we have all dependencies.
 set(ENABLE_CHECK_TARGETS TRUE)
 
 # Function to find required dependencies for testing.
 function(find_standalone_test_dependencies)
-  include(FindPythonInterp)
+  find_package (Python3 COMPONENTS Interpreter)
 
-  if (NOT PYTHONINTERP_FOUND)
+  if (NOT Python3_Interpreter_FOUND)
     message(STATUS "Could not find Python.")
     message(WARNING "The check targets will not be available!")
     set(ENABLE_CHECK_TARGETS FALSE PARENT_SCOPE)
     return()
   endif()
 
   # Find executables.
   find_program(OPENMP_LLVM_LIT_EXECUTABLE
if (NOT ARG_EXCLUDE_FROM_CHECK_ALL)
     # Register the testsuites and depends for the check-openmp rule.
     set_property(GLOBAL APPEND PROPERTY OPENMP_LIT_TESTSUITES ${ARG_UNPARSED_ARGUMENTS})
     set_property(GLOBAL APPEND PROPERTY OPENMP_LIT_DEPENDS ${ARG_DEPENDS})
   endif()
 
   if (${OPENMP_STANDALONE_BUILD})
     set(LIT_ARGS ${OPENMP_LIT_ARGS} ${ARG_ARGS})
     add_custom_target(${target}
-      COMMAND ${PYTHON_EXECUTABLE} ${OPENMP_LLVM_LIT_EXECUTABLE} ${LIT_ARGS} ${ARG_UNPARSED_ARGUMENTS}
+      COMMAND ${Python3_EXECUTABLE} ${OPENMP_LLVM_LIT_EXECUTABLE} ${LIT_ARGS} ${ARG_UNPARSED_ARGUMENTS}
       COMMENT ${comment}
       DEPENDS ${ARG_DEPENDS}
       USES_TERMINAL
     )
   else()
     if (ARG_EXCLUDE_FROM_CHECK_ALL)
       add_lit_testsuite(${target}
         ${comment}