contrib: Use text=True in subprocess over manual encoding handling

All touched Python scripts already assume and require UTF8, so manually
specifying encoding or decoding for functions in the subprocess module
is redundant to just using text=True, which exists since Python 3.7

test/lint/lint-include-guards.py

@@ -31,7 +31,7 @@ def _get_header_file_lst() -> list[str]:
     """
     git_cmd_lst = ['git', 'ls-files', '--', '*.h']
     header_file_lst = check_output(
-        git_cmd_lst).decode('utf-8').splitlines()
+        git_cmd_lst, text=True).splitlines()
 
     header_file_lst = [hf for hf in header_file_lst
                        if not any(ef in hf for ef