Add explicit language_level=3 arg to Cythonize

[jrmccluskey]

Adds an explicit lanugage_level=3 arg to Cythonize() in an effort to suppress warnings and avoid accidental usage of Cythonize targeting Python 2.

Fixes #25979

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[damccorm]

LGTM

[jrmccluskey]

Run Python PreCommit

[robertwb]

Thanks for taking care of this.

[jrmccluskey]

Confirmed that the cython tests in the precommit no longer have the warning. Will let everything go green before merging

[jrmccluskey]

Run PythonLint PreCommit

[jrmccluskey]

Run RAT PreCommit

[jrmccluskey]

Run Portable_Python PreCommit

[jrmccluskey]

Everything got cancelled by Jenkins restarting but we've confirmed the fix so I'm comfortable merging