Adding Flake8 F4 Tests to find Import Errors

[PatOnTheBack]

@PatOnTheBack if you add F4 to the flake8 tests in .travis.yml, you will find other import-related issues to fix. Thanks for your attention to these details.

Originally posted by @cclauss in #920 (comment)

Like cclauss said, adding Flake8 F4 to the Travis file can help us find import errors that reduce code qulaity.

[cclauss]

https://github.com/TheAlgorithms/Python/blob/master/.travis.yml#L8