[FLINK-38094][python] Introduce pyflink.__version__

[Myasuka]

What is the purpose of the change

Introduce pyflink.__version__ method, which is widely provided by many popular python libraries.

Brief change log

Introduce pyflink.__version__ method.

Verifying this change

Please make sure both new and modified tests in this PR follow the conventions for tests defined in our code quality guide.

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[Myasuka]

@dianfu would you please help take a review?

[flinkbot]

CI report:

• ea7e318 Azure: SUCCESS

Bot commands

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• @flinkbot run azure re-run the last Azure build

[dianfu]

@Myasuka +1. LGTM. Thanks for the PR!