maths/radix2_fft.py: Fix calculation for Python 3.14

[cclauss]

Fixes: #12729

As suggested at:

• Help wanted: Why do maths/radix2_fft.py test results change on Python 3.14 beta? #12729 (comment)
• Test Python 3.14 #12710 (comment)

@MaximSmolskiy@KirilBangachev@skirpichev Careful review, please. I am just implementing the suggestion, but I do not have any expertise in this area.

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.
• 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]

It is too confusing to have two GitHub Actions jobs named build, so rename this job to document what it really does.

[MaximSmolskiy]

I have a basic knowledge of complex numbers from university course. For me, the changes are equivalent. So, the difference lies somewhere deep in the subtleties of practical implementation, which I don't know at all

[cclauss]

Nice teamwork on this one. Thanks much!