Problem

pull_request.create_review_request(reviewers="some-reviewer") raises a GithubException if a single reviewer is provided as a string. This occurs because GitHub expects the reviewers property to be an array, not a single string (Github docs).

Exception example:

github.GithubException.GithubException: 422 {"message": "Invalid request.
For 'properties/reviewers', \"some-reviewer\" is not an array.", "documentation_url": "https://docs.github.com/rest/pulls/review-requests#request-reviewers-for-a-pull-request", "status": "422"}

Fix

Wrap the string arguments (reviewers, team_reviewers) into a list when a single reviewer is provided. This ensures compatibility with GitHub's API, which expects the reviewers property to always be an array.

Fixes #3089