Complex numbers operations: sum, mul, abs

[Adri-md-1208]

Describe your change:

I create a complex/ directory inside the math/ category because there are no algorithms for complex numbers and I think that is a good idea to separate the basic functions of complex and non complex numbers like addition, multiplication or absolute value.
I also create these last three algorithms for complex numbers.

Checklist:

• Add an algorithm?
• 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}.
  This PR is aimed to contribute to Hacktoberfest 2021 - Implement algorithms or data structures, fix existing ones and more! #4996

[ghost]

Click here to look at the relevant links ⬇️

🔗 Relevant Links

Repository:

• Contributing guidelines
• Project Euler solution guidelines

Python:

• Formatted string literals (f-strings)
• Type hints
• doctest
• unittest
• pytest

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[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 do not have passing tests.