Refactor mixed_keyword_cypher.py file and added comments in code

[Ranausama420]

Describe your change:

changed below lines added sets functionality and remove for loops to get unique element in list
temp = list(set(key))
len_temp = len(temp)

string module can used to generate alphabets list instead of using loops
alpha = list(string.ascii_uppercase)

and used string join method to generate cypher

Resolves #8622

• 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 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 commit message contains Fixes: #{$ISSUE_NO}.

[tianyizheng02]

This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.

[ankit-gautam23]

Effective changes with neat one liner comment. I don't think it gonna break anywhere in the system.

[tianyizheng02]

1. As @CaedenPH said in your duplicate PR Documentation #8646, the documentation doesn't add much value because it just talks through the code, with most of the info easily inferable from the code itself.
2. This issue was already resolved in Improve readability of ciphers/mixed_keyword_cypher.py #8626.