[CI] Pin cython version to fix cython compilation

[lhutton1]

Cython 3.0.0 was recently released, but it is incompatible with the current .pxi definitions in python/tvm/_ffi/_cpython. Pinning the cython version until a working fix is created.

Posting this PR as an alternative to #15346.

[tvm-bot]

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• cc @Mousius, @areusch, @leandron _{See #10317 for details}

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[Mousius]

This seems sensible given updating in Conda likely requires a bit of effort

[tqchen]

I think it is better to see if we can get #15346 through, we could remove 3.7 requirement from the conda there

[junrushao]

Instead of pinning package to a lower version, we might want to fix this particular issue instead to get consistent CI vs user/developer-end behavior

We tried, we couldn't make it work, preference is not to block this PR which fixes the immediate issue.

[Mousius]

@tqchen / @junrushao in an ideal world, we would fix everything. As you can see from #15346, it's not straightforward due to how neglected Conda is. I will merge this to resolve the issue and get things back into a stable state; if you can correct Conda and update Cython, that'd be great, we lack the expertise in this area 😸