Apply isort to pl_examples/

[akihironitta]

What does this PR do?

Part of #4805.

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes [if needed]?
• Did you write any new necessary tests [no need for typos, docs]?
• Did you verify new and existing tests pass locally with your changes?
• If you made a notable change (that affects users), did you update the CHANGELOG?

PR review

Anyone in the community is free to review the PR once the tests have passed.
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Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #5291 (0a91377) into master (eb1d61c) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #5291   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         134     134           
  Lines        9950    9950           
======================================
  Hits         9261    9261           
  Misses        689     689