MAINT: Use explicitly public APIs

[larsoner]

In mne-tools/mne-python#11903 we're trying to make some mne/io stuff private. This changes mne-nirs to use the explicitly public API

Also includes a number of fixes for latest NumPy/SciPy/Pandas

[codecov]

Codecov Report

Merging #521 (d40efea) into main (b518247) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #521      +/-   ##
==========================================
- Coverage   95.28%   95.22%   -0.07%     
==========================================
  Files          69       69              
  Lines        2865     2868       +3     
  Branches      412      412              
==========================================
+ Hits         2730     2731       +1     
- Misses         65       66       +1     
- Partials       70       71       +1     

Files Changed	Coverage Δ
mne_nirs/conftest.py	96.49% <ø> (ø)
mne_nirs/preprocessing/tests/test_mayer.py	100.00% <100.00%> (ø)
mne_nirs/statistics/_glm_level_first.py	88.58% <100.00%> (+0.78%)	⬆️
mne_nirs/statistics/_statsmodels.py	91.02% <100.00%> (+0.11%)	⬆️
mne_nirs/statistics/tests/test_glm_type.py	98.99% <100.00%> (-1.01%)	⬇️
mne_nirs/tests/test_examples.py	80.64% <100.00%> (-6.86%)	⬇️
mne_nirs/utils/_io.py	96.25% <100.00%> (ø)
mne_nirs/visualisation/_plot_3d_montage.py	98.01% <100.00%> (-0.02%)	⬇️
mne_nirs/visualisation/_plot_GLM_topo.py	85.23% <100.00%> (ø)

[drammock]

nitpicks (I like examples to show what we consider best practices)