test: migrate tests to Python3 by default

This changes the python interpreter used to invoke the tests to Python3.
Python2 has been EOL'ed by the Python Software Foundation.  This
migration enables the Swift test suite to run with Python 3 instead.

Author: compnerd

test/CMakeLists.txt

@@ -351,7 +351,7 @@ _Block_release(void) { }\n")
       endif()
 
       execute_process(COMMAND
-          $<TARGET_FILE:Python2::Interpreter> "-c" "import psutil"
+          $<TARGET_FILE:Python3::Interpreter> "-c" "import psutil"
           RESULT_VARIABLE python_psutil_status
           TIMEOUT 1 # second
           ERROR_QUIET)
@@ -415,7 +415,7 @@ _Block_release(void) { }\n")
             ${command_upload_swift_reflection_test}
             ${command_clean_test_results_dir}
             COMMAND
-              $<TARGET_FILE:Python2::Interpreter> "${LIT}"
+              $<TARGET_FILE:Python3::Interpreter> "${LIT}"
               ${LIT_ARGS}
               "--param" "swift_test_subset=${test_subset}"
               "--param" "swift_test_mode=${test_mode}"
@@ -434,7 +434,7 @@ _Block_release(void) { }\n")
             ${command_upload_swift_reflection_test}
             ${command_clean_test_results_dir}
             COMMAND
-              $<TARGET_FILE:Python2::Interpreter> "${LIT}"
+              $<TARGET_FILE:Python3::Interpreter> "${LIT}"
               ${LIT_ARGS}
               "--param" "swift_test_subset=${test_subset}"
               "--param" "swift_test_mode=${test_mode}"