[FLINK-22733][python] DataStream.union should handle properly for KeyedStream in Python DataStream API

[dianfu]

What is the purpose of the change

This pull request fixes DataStream.union which should consider if the given DataStream is KeyedStream in Python DataStream API

Verifying this change

This change added tests and can be verified as follows:

(example:)

• Added test case test_keyed_stream_union

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/Mesos, 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)

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Automated Checks

Last check on commit e273171 (Sat Aug 28 12:11:34 UTC 2021)

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[HuangXingBo]

+1

[flinkbot]

CI report:

• e273171 Azure: SUCCESS

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