Tiletool: Switch str to bytes.

[videlanicolas]

When padding a sprite with 0x00, tiletool.py is erroring out with:

Traceback (most recent call last):
  File "/workspaces/ngdevkit/tools/./tiletool.py", line 379, in <module>
    main()
  File "/workspaces/ngdevkit/tools/./tiletool.py", line 375, in main
    conv.create()
  File "/workspaces/ngdevkit/tools/./tiletool.py", line 151, in create
    self.close_rom()
  File "/workspaces/ngdevkit/tools/./tiletool.py", line 245, in close_rom
    self.fd1.write('\0'*padding)
TypeError: a bytes-like object is required, not 'str'