Added doctests for fibonacci.py

[nkstonks]

Describe your change:

#9943

Added doctests for the fib_recursive_term function in maths/fibonacci.py

• 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.
• 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 include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[nkstonks]

Hold on, doctest seems to not be testing the function in question, I believe it's because it's a nested function. I'll need to figure something out for that.

Sorry for not checking this properly.

[nkstonks]

I closed this pull request as I realised doctest wasn't properly testing the function (fib_recursive_term) I implemented doctests for.

However I found out that doctests doesn't test local functions, which was the function I implemented doctests for.

So I'm reopening this pull request to ask whether we need to implement a way doctest will actually test that function, or just merge it anyways as the doctests are working (I've tested the function in a separte isolated file).