Add GitHub action for pre-commit

[dhruvmanila]

Follow-up on #2511
As mentioned in #2511 (comment)

Describe your change:

• Add an algorithm?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.

[dhruvmanila]

It's working perfectly: https://github.com/TheAlgorithms/Python/pull/2515/checks?check_run_id=1186742542

A few small changes required:

• Fix instagram_crawler.py using black
• Either add shebang or remove the executable bit from those files

With this in place, we can remove black, flake8, validate_filenames.py step from Travis CI and it needs to check only with pytest.

[dhruvmanila]

I think the name of the file should be lint, what do you think? @cclauss

[cclauss]

Simplifying by always using the latest and greatest Python and by dropping names that are less self-documenting than the commands that they run.

[cclauss]

Why not just add the shebang instead of removing the execution bit?

[cclauss]

I think the name of the file should be lint, what do you think? @cclauss

I think the current name is good enough. The script runs pre-commit so it should be called pre-commit.

[dhruvmanila]

Except for the scheduling/round_robin.py file, other files don't output anything when I run python <filename> so I thought there's no point in keeping the executable bit on.

If you think otherwise, I will turn them on?