Simplify diffie_hellman.py

[isidroas]

Describe your change:

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

[isidroas]

These are many unrelated changes. Let me know if I have to divide them into commits or PRs.

[isidroas]

The git history shows that the author is @ruppysuppy . Do these changes seem good to you?

Everything is a refactor except the method generate_shared_key(): its hashing final step has been removed for simplicity and because the key derivation is a separate problem.

[isidroas]

Thank you for the feedback @ruppysuppy ! I added a bit of documentation in 93c7ac0116508475e452e2027d700f0648b6f5d6