Support Cython 3

[musicinmybrain]

Fix a relative cimport and revert the commit that pinned cython<3.0.

[skjerns]

I assume #233 and #234 has been fixed?

[musicinmybrain]

I assume #233 and #234 has been fixed?

Yes, I found that everything was fine (it compiled and the tests passed) after I fixed the cimport:

      cimport c_edf
              ^
      ------------------------------------------------------------
      
      pyedflib/_extensions/_pyedflib.pyx:24:8: 'c_edf.pxd' not found

[skjerns]

Strange, I could have sworn that I tried that.

Thanks! That fixes it :)