[Bug]: Cythonization is compiled with language level 2

[Abacn]

What happened?

Test logs show many warnings from cython compiler (e.g. https://ci-beam.apache.org/job/beam_PreCommit_Python_Cron/6886/consoleFull)

03:09:03 /home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Cron/src/sdks/python/test-suites/tox/py38/build/srcs/sdks/python/target/.tox-py38-cython/py38-cython/lib/python3.8/site-packages/Cython/Compiler/Main.py:369:
FutureWarning: Cython directive 'language_level' not set, using 2 for now (Py2).
This will change in a later release!
File: /home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Cron/src/sdks/python/test-suites/tox/py38/build/srcs/sdks/python/apache_beam/transforms/stats.pxd

Fortunately it still works so far. Nevertheless we should explicit to use language-level 3.

Found when experimenting #25970

Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

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