Add doctests for fractional knapsack

[Hardvan]

Added doctests for the Fractional Knapsack algorithm, as requested in PR #10888

• 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".

[cclauss]

On your computer do python3 -m doctest -v filename.py to run the tests.

[Hardvan]

I have successfully executed the doctests, yielding the following output:

11 tests in fractional_knapsack.frac_knapsack
11 tests in 2 items.
11 passed and 0 failed.
Test passed.

[Hardvan]

After adding this test case, this is the output:
r = sorted(zip(vl, wt), key=lambda x: x[0] / x[1], reverse=True)
TypeError: unsupported operand type(s) for /: 'str' and 'int'

---

1 items had failures:
1 of 12 in main.frac_knapsack
Test Failed 1 failures.

This suggests that the existing code cannot handle the case when the input parameters are of another unexpected type (string).