Add negative int support to 3n+1

[Rudxain]

Describe your change:

Now the n31 algorithm has been extended to support negative integers, and has protection against infinite looping by checking for 0. The algorithm is still simple enough, so it's still useful for educational purposes

• 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}.

[Rudxain]

What went wrong? It seems it's the formatting, but where?

[Rudxain]

For some reason, it seems like I accidentally deleted some digits from the file while editing. It should be fixed now

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[cclauss]

In preparation for that denial-of-service attack that is Hacktoberfest, I am closing all pull requests that have failing tests.

[Rudxain]

What? But (AFAIK) the (new) failing tests were not my fault, it was from upstream. I don't understand

[Rudxain]

@cclauss can I (and should I) open a new PR?