Fix Python3 (Windows) compatibility (#15597)

Marlin/src/HAL/HAL_LPC1768/upload_extra_script.py

         #
         import subprocess
         # typical result (string): 'Drives: C:\ D:\ E:\ F:\ G:\ H:\ I:\ J:\ K:\ L:\ M:\ Y:\ Z:\'
-        driveStr = subprocess.check_output("fsutil fsinfo drives")
+        driveStr = subprocess.check_output("fsutil fsinfo drives").decode('utf8')
         # typical result (string): 'C:\ D:\ E:\ F:\ G:\ H:\ I:\ J:\ K:\ L:\ M:\ Y:\ Z:\'
         driveStr = driveStr.strip().lstrip('Drives: ')
         # typical result (array of stings): ['C:\\', 'D:\\', 'E:\\', 'F:\\',
         for drive in drives:
             final_drive_name = drive.strip().rstrip('\\')   # typical result (string): 'C:'
             try:
-                volume_info = subprocess.check_output('cmd /C dir ' + final_drive_name, stderr=subprocess.STDOUT)
+                volume_info = subprocess.check_output('cmd /C dir ' + final_drive_name, stderr=subprocess.STDOUT).decode('utf8')
             except Exception as e:
                 continue
             else: