Warning: Discarding `const` from pointers in `fletcher32`

[jakirkham]

Seeing warnings about discarding const from pointers in fletcher32 on CI :

  building 'numcodecs.fletcher32' extension
  /home/runner/miniconda3/envs/test/bin/aarch64-conda-linux-gnu-cc -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O3 -pipe -isystem /home/runner/miniconda3/envs/test/include -DNDEBUG -D_FORTIFY_SOURCE=2 -O2 -isystem /home/runner/miniconda3/envs/test/include -fPIC -Inumcodecs -I/home/runner/miniconda3/envs/test/include/python3.12 -c numcodecs/fletcher32.c -o /tmp/tmpp_01vogx.build-temp/numcodecs/fletcher32.o
  numcodecs/fletcher32.c: In function '__pyx_pf_9numcodecs_10fletcher32_10Fletcher32_encode':
  numcodecs/fletcher32.c:18290:17: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  18290 |   __pyx_v_b_ptr = (&(*((uint8_t const  *) ( /* dim=0 */ ((char *) (((uint8_t const  *) __pyx_v_b_mv.data) + __pyx_t_8)) ))));
        |                 ^
  numcodecs/fletcher32.c: In function '__pyx_pf_9numcodecs_10fletcher32_10Fletcher32_2decode':
  numcodecs/fletcher32.c:18649:17: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  18649 |   __pyx_v_b_ptr = (&(*((uint8_t const  *) ( /* dim=0 */ ((char *) (((uint8_t const  *) __pyx_v_b_mv.data) + __pyx_t_7)) ))));
        |                 ^