MAINT: Numpy 2.0 compat

[larsoner]

Should fix https://dev.azure.com/mne-tools/mne-python/_build/results?buildId=26981&view=logs&jobId=dded70eb-633c-5c42-e995-a7f8d1f99d91&j=dded70eb-633c-5c42-e995-a7f8d1f99d91&t=d18f7f2f-13af-5901-1cbc-7fa039d0db3a

================================== FAILURES ===================================
________________________________ test_birthday ________________________________
mne\io\snirf\tests\test_snirf.py:475: in test_birthday
    snirf = pytest.importorskip("snirf")
C:\hostedtoolcache\windows\Python\3.11.5\x64\Lib\site-packages\snirf\__init__.py:1: in <module>
    from .pysnirf2 import *
C:\hostedtoolcache\windows\Python\3.11.5\x64\Lib\site-packages\snirf\pysnirf2.py:145: in <module>
    _STR_DTYPES = [str, np.string_]
C:\hostedtoolcache\windows\Python\3.11.5\x64\Lib\site-packages\numpy\__init__.py:397: in __getattr__
    raise AttributeError(
E   AttributeError: `np.string_` was removed in the NumPy 2.0 release. Use `np.bytes_` instead.