Compress fails on binary string
Compress fails with:
```shell
ValueError: '■' is not in list
```
or:
```shell
---> 11 write_cart("carts/new_cart.p8.rom", new_cart, CartFormat.rom)

File [c:\Users\Frederic\Source\Repos\hexen\.venv\lib\site-packages\pico_cart.py:711](file:///C:/Users/Frederic/Source/Repos/hexen/.venv/lib/site-packages/pico_cart.py:711), in write_cart(path, cart, format, **opts)
    709     file_write(path, write_cart_to_image(cart, **opts))
    710 elif format == CartFormat.rom:
--> 711     file_write(path, write_cart_to_rom(cart, **opts))
    712 elif format == CartFormat.tiny_rom:
    713     file_write(path, write_cart_to_tiny_rom(cart, **opts))

File [c:\Users\Frederic\Source\Repos\hexen\.venv\lib\site-packages\pico_cart.py:101](file:///C:/Users/Frederic/Source/Repos/hexen/.venv/lib/site-packages/pico_cart.py:101), in write_cart_to_rom(cart, with_trailer, keep_compression, **opts)
     99     w.bytes(cart.code_rom)
    100 else:
--> 101     compress_code(w, cart.code, **opts)
    103 if with_trailer:
    104     w.setpos(k_cart_size)
...
    475         w.u16(size & 0xffff) # only throw under fail_on_error above
    477 else:
--> 478     w.bytes(bytes(ord(c) for c in code))

ValueError: bytes must be in range(0, 256)
```

Repro code:
```python
from pico_cart import write_cart, CartFormat, Cart
new_cart = Cart()
new_cart.rom[0x0000:0x2000] = bytearray(0x2000) # zero it out
new_cart.set_code("""-- my game
-- @freds72
local title_gfx={bytes="\0\0\0\0\0\0\0\0\0¹\0\0\0■\0\0■▮\0■■■■■■■■"}
function _init()
end
""")

write_cart("carts/new_cart.p8.rom", new_cart, CartFormat.rom)
```

note: the binary string has been copied from Pico (eg. valid)