build: switch gyb to Python3

Change the build system to invoke gyb with python3 instead of python2.

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -23,7 +23,7 @@ function(_swift_gyb_target_sources target scope)
 
     add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${generated}
       COMMAND
-        $<TARGET_FILE:Python2::Interpreter> ${SWIFT_SOURCE_DIR}/utils/gyb -D CMAKE_SIZEOF_VOID_P=${CMAKE_SIZEOF_VOID_P} ${SWIFT_GYB_FLAGS} -o ${CMAKE_CURRENT_BINARY_DIR}/${generated}.tmp ${absolute}
+        $<TARGET_FILE:Python3::Interpreter> ${SWIFT_SOURCE_DIR}/utils/gyb -D CMAKE_SIZEOF_VOID_P=${CMAKE_SIZEOF_VOID_P} ${SWIFT_GYB_FLAGS} -o ${CMAKE_CURRENT_BINARY_DIR}/${generated}.tmp ${absolute}
       COMMAND
         ${CMAKE_COMMAND} -E copy_if_different ${CMAKE_CURRENT_BINARY_DIR}/${generated}.tmp ${CMAKE_CURRENT_BINARY_DIR}/${generated}
       COMMAND

cmake/modules/SwiftHandleGybSources.cmake

@@ -1,8 +1,6 @@
 include(SwiftAddCustomCommandTarget)
 include(SwiftSetIfArchBitness)
 
-find_package(Python2 COMPONENTS Interpreter REQUIRED)
-
 # Create a target to process single gyb source with the 'gyb' tool.
 #
 # handle_gyb_source_single(
@@ -60,7 +58,7 @@ function(handle_gyb_source_single dependency_out_var_name)
       COMMAND
           "${CMAKE_COMMAND}" -E make_directory "${dir}"
       COMMAND
-          "$<TARGET_FILE:Python2::Interpreter>" "${gyb_tool}" ${SWIFT_GYB_FLAGS} ${GYB_SINGLE_FLAGS} -o "${GYB_SINGLE_OUTPUT}.tmp" "${GYB_SINGLE_SOURCE}"
+          "$<TARGET_FILE:Python3::Interpreter>" "${gyb_tool}" ${SWIFT_GYB_FLAGS} ${GYB_SINGLE_FLAGS} -o "${GYB_SINGLE_OUTPUT}.tmp" "${GYB_SINGLE_SOURCE}"
       COMMAND
           "${CMAKE_COMMAND}" -E copy_if_different "${GYB_SINGLE_OUTPUT}.tmp" "${GYB_SINGLE_OUTPUT}"
       COMMAND