[FLINK-17068][python][tests] Ensure the permission of scripts set correctly before executing the Python tests

[dianfu]

What is the purpose of the change

This pull request add the executable permission for the Python scripts before executing the Python tests.

Brief change log

• Ensure the permission of scripts set correctly in dev/lint-python.sh

Verifying this change

Verify manually in my local environment.

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 2670def (Sun Apr 19 04:32:11 UTC 2020)

Warnings:

• No documentation files were touched! Remember to keep the Flink docs up to date!

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Review Progress

• ❓ 1. The [description] looks good.
• ❓ 2. There is [consensus] that the contribution should go into to Flink.
• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

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CI report:

• 2a45932 Travis: SUCCESS Azure: SUCCESS

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[rmetzger]

+1 to merge once the Azure tests have passed

[dianfu]

@rmetzger Thanks a lot for the review. I will merge it once the tests passed.