Adding reverse_letters.py

[Snimer]

Describe your change:

Added reverse_letters.py
It reverses the letters of words of the string without adjusting the position of words

• 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}.

[TravisBuddy]

Travis tests have failed

Hey @Snimer,
Please read the following log in order to understand the failure reason.
It'll be awesome if you fix what's wrong and commit the changes.

TravisBuddy Request Identifier: 903c9480-112a-11eb-bfd1-1f84b849c73d

[TravisBuddy]

Travis tests have failed

Hey @Snimer,
Please read the following log in order to understand the failure reason.
It'll be awesome if you fix what's wrong and commit the changes.

TravisBuddy Request Identifier: 3ae9d140-112b-11eb-bfd1-1f84b849c73d

[realDuYuanChao]

Bro. You delete a branch of this PR. Please create a new PR.

[Snimer]

Bro. You delete a branch of this PR. Please create a new PR.

@shellhub
Created a new PR-
#3730