Fix MNE bdf/gdf EEG file imports

[Teteros]

Importing .bdf and .gdf currently fails (NME 0.21.1) because those filetypes have their own importers.
This change will make them work again in the Sleep GUI when chosen on open file.

[raphaelvallat]

Hi @Teteros,

Thank you so much for the PR, and so sorry for the late reply...! The PR looks great. Questio: does that mean that Visbrain will require MNE>0.21.1?

Also worth noting, since MNE 0.22 there is a generic function (https://mne.tools/stable/generated/mne.io.read_raw.html#mne.io.read_raw) that automatically call the appropriate function based on the file extension. But I would prefer to keep compatibility with older versions of MNE for now (at least 0.21).

Thanks
Raphael

[Teteros]

does that mean that Visbrain will require MNE>0.21.1?

The minimum version of MNE required for this change is 0.18 I believe
https://github.com/mne-tools/mne-python/blob/main/doc/changes/0.18.inc#L258

as that's when the edf reader was split into BDF/GDF ones.