Fixup tests on Windows, use UTF-8 for subprocess

tests/unit/session/test_parallel.py

@@ -270,9 +270,11 @@ def test_parallel_result_json_concurrent(cmd, parallel_project, tmp_path):
     def invoke_tox_in_thread(thread_name, result_json):
         try:
             # needs to be process to have it's own stdout
+            kwargs = {"encoding": "utf-8"} if sys.version_info[0] > 2 else {}
             invoke_result[thread_name] = subprocess.check_output(
                 [sys.executable, "-m", "tox", "-p", "all", "--result-json", str(result_json)],
                 universal_newlines=True,
+                **kwargs
             )
         except subprocess.CalledProcessError as exception:
             invoke_result[thread_name] = exception