Resolve the issue: prefer single quote strings

[rshiue]

What does this pull request do?

In order to prefer single quote to make consistent with Tensorflow codebase, replace double-quote with single-quote in Python files.

Fixes #279

How did you test this change?

Run pytest and pre-commit hooks(style checks)

How did you document this change?

No system behaviors got changed at all.
Just code style changes.

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