Create youtube_downloader.py

[s18k]

Python program to download a Youtube video as a .mp4 file

Describe your change:

• [x] Add an algorithm?
• [ ] Fix a bug or typo in an existing algorithm?
• [ ] Documentation change?

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.
• [ ] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [ ] All new Python files are placed inside an existing directory.
• [ ] All filenames are in all lowercase characters with no spaces or dashes.
• [ ] All functions and variable names follow Python naming conventions.
• [ ] All function parameters and return values are annotated with Python type hints.
• [ ] All functions have doctests that pass the automated testing.
• [ ] All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• [ ] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[dhruvmanila]

@algorithms-keeper review

[cclauss]

web_programming/youtube_downloader.py:4: in <module>
    from pytube import YouTube
E   ModuleNotFoundError: No module named 'pytube'

---

web_programming/youtube_downloader.py:1:1: F401 'os' imported but unused web_programming/youtube_downloader.py:11:5: F841 local variable 'filename' is assigned to but never used

[dhruvmanila]

Testing out whether the bot is ignoring to check doctest existence in web_programming/ directory:

@algorithms-keeper review

[dhruvmanila]

Hmm, not working, will take a look in a few hours.

[dhruvmanila]

@algorithms-keeper review

[dhruvmanila]

Working!