Adopt CMP0148: Remove Deprecated PythonInterp

CMake deprecated the PythonInterp package a while ago now, so running
`find_package(PythonInterp)` emits warnings saying that it's being
removed. This patch switches to the newer `Python3` package.

The llvm-lit binary makes use of newer language features that are only
available in Python 3.7 and newer, so I've also bumped the minimum
required Python version to avoid issues there.

Author: etcwilde

CMakeLists.txt

@@ -88,7 +88,7 @@ set(LLBUILD_LIBDIR_SUFFIX "${LIB_SUFFIX}" CACHE STRING "Set default library fold
 if(BUILD_TESTING)
   find_package(Lit)
   find_package(FileCheck)
-  find_package(PythonInterp)
+  find_package(Python3 3.7 COMPONENTS Interpreter)
 endif()
 
 ###

tests/CMakeLists.txt

@@ -18,8 +18,7 @@ configure_file(
   ${CMAKE_CURRENT_SOURCE_DIR}/Unit/lit.site.cfg.in
   ${CMAKE_CURRENT_BINARY_DIR}/Unit/lit.site.cfg)
 
-include(FindPython)
-if(Python_Interpreter_FOUND AND LIT_FOUND AND FILECHECK_FOUND)
+if(Python3_Interpreter_FOUND AND LIT_FOUND AND FILECHECK_FOUND)
   set(LIT_ARGS "${LLBUILD_LIT_ARGS}")
   separate_arguments(LIT_ARGS)
 
@@ -29,7 +28,7 @@ if(Python_Interpreter_FOUND AND LIT_FOUND AND FILECHECK_FOUND)
   endif()
 
   set(lit_command
-    ${Python_EXECUTABLE}
+    ${Python3_EXECUTABLE}
     ${LIT_EXECUTABLE}
     ${LIT_ARGS}
     --param build_mode=${build_mode})