[bugfix] Compatibility with NumPy >=2.5

[DrYak]

• Recent Numpy version only allow specifying array-protocol data type for nul-terminated bytes using S
• the a alias as used in pysamstat is not valid anymore

This caused my software to crash with:

  File "…/lib/python3.12/site-packages/pysamstats/pileup.py", line 140, in load_pileup
    return util.load_stats(statfun, user_dtype=dtype, default_dtype=default_dtype,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "…/lib/python3.12/site-packages/pysamstats/util.py", line 67, in load_stats
    a = np.fromiter(it, dtype=dtype)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This patch changes the specifer to S