Update atbash cipher (doc, doctest, performance)

[l3str4nge]

Describe your change:

Added update for atbash cipher:

• doctests,
• type hints
• little performance improvement

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• 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 have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[cclauss]

Running performance benchmarks...
> atbash_slow() 18.921338292999998 seconds
>      atbash() 16.922152821999997 seconds

How could atbash() be modified to be much faster?

[l3str4nge]

With small string speed is not visible - even worse. I've tried with 4 000 000 characters length and current implementation is little bit faster (from 3.2 sec -> 2.2 - 2.4 sec). That's why I wrote "little" in PR description :)

I can put some performance tests I made If you want :)

[l3str4nge]

Running performance benchmarks...
> atbash_slow() 18.921338292999998 seconds
>      atbash() 16.922152821999997 seconds

How could atbash() be modified to be much faster?

I thought about modulo operations and remove completely searching characters in ascii_letters. If statement in list comprehensions slowing down too. For now I don't have idea but I know it's possible :)

[cclauss]

Would it help if we moved the definition of letters and letters_reversed outside the function?

[l3str4nge]

Would it help if we moved the definition of letters and letters_reversed outside the function?

Perhaps but insignifficant, these strings are too small. The bottleneck is here:
.. letters.index(c) .. if c in letters else c. ..