Created factorial.py

[singhvibhanshu]

Fixes #12142

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
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• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
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[algorithms-keeper]

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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[algorithms-keeper]

As there is no test file in this pull request nor any test function or class in the file recursion/factorial.py, please provide doctest for the function factorial

Please provide return type hint for the function: factorial. If the function does not return a value, please provide the type hint as: def function() -> None:

Please provide descriptive name for the parameter: n

Please provide type hint for the parameter: n

[singhvibhanshu]

def print_butterfly -> None:
n is used to ask the user to enter the length of butterfly

[cclauss]

Closing require_type_hints PRs to prepare for Hacktoberfest