merge sort challenge in python

[abed42]

• [ ] PR's title includes the challenge's name and language
• [ ] PR has short and clear description of the challenge
• [ ] PR has appropriate labels and milestones for easy identification
• [ ] PR it is assigned to the owner
• [ ] reviewers are assigned
• [ ] the PR contributes only one focused change
• [ ] the branch is up to date with main/master
• [ ] the code works when pulled and run locally
• [ ] all conflicts are resolved (if any)
• [ ] It is linked to an issue in the appropriate column of the project board
• [ ] feedback is addressed (if any, and if it is appropriate feedback.)

README Documentation

• [ ] The solution is documented in /src/README.md
• [ ] The markdown source is formatted
• [ ] Spelling and grammar is correct in all text
• [ ] The markdown looks correct when you preview the file
• [ ] All links and images work
• [ ] The README documents the solution's behavior, strategy(ies) and implementation(s)
• [ ] There are use cases in code blocks to illustrate the function's behavior

Python Files

• [ ] There is a module header
• [ ] There is a module docstring
• [ ] File names are in snake_case
• [ ] Test files are named test_<module_name>.py

Function Docstring

• [ ] Behavior description
• [ ] Parameter description
• [ ] Return value description
• [ ] Include any assumptions
• [ ] Include 3 or more (passing!) doctests
• [ ] Include 1-2 use cases (if necessary)

Function Implementation

• [ ] The solution is not is not copied from djeada
• [ ] The code is auto-formatted
• [ ] The code has no (reasonable) linting mistakes
• [ ] Variables are named with snake_case
• [ ] The function has a clear and helpful name
• [ ] The file's name matches the function name
• [ ] The code follows the strategy as simply as possible
• [ ] Variable names are clear and helpful
• [ ] Comments explain the strategy (if necessary)
• [ ] There are type annotations
• [ ] (challenge) The code includes defensive assertions

Unit Test Suite

• [ ] The test class has a helpful name in PascalCase
• [ ] The test class has a docstring
• Each unit test has
  • [ ] A helpful name
  • [ ] A clear docstring
  • [ ] Only one assertion
• [ ] All tests pass
• [ ] There are tests for defensive assertions
• [ ] There are tests for boundary cases
• [ ] Test suite includes black-box and glass-box tests