Change setup.py to work with Cython 3.0

[janezd]

Issue

Fixes #6519.

Cython 3.0 with current setup.py cythonizes pyx -> c, but doesn't call compiler.

Description of changes

Call cythonize. Whyever.

I checked that there should be no problems with new default behaviour of / ("true division", producing floats). In most pyx files we specifiy cdivision=True anyway, so the behaviour of Cython remained the same).

If this is OK, we need to

• also merge Fix setup to work with Cython 3.0 orange3-network#253
• check other officially supported extensions that use Cython; I checked those I have have installed)

Interestingly, with addon Associate we distribute .cpp. When was the last time anybody touched that add-on?

[codecov]

Codecov Report

Merging #6521 (c6b6d4e) into master (601b7dc) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6521   +/-   ##
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  Coverage   87.68%   87.68%           
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  Files         321      321           
  Lines       69371    69371           
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  Hits        60830    60830           
  Misses       8541     8541           

[markotoplak]

If we call cythonize manually I see no reason to use cython's build_ext.

I have no idea why this PR changes anything. @ales-erjavec, could you comment on this PR?