Add support for Subreddit Removal Reasons

[Dan6erbond]

Closes #94

Feature Summary

Explain what's new in this pull request.

class SubredditRemovalReasons / class SubredditRemovalReason:

• SubredditRemovalReasons.get(item: str) to retrieve a removal reason by ID.
• Implement __aiter__ and __anext__ to be used as an async generator.
• SubredditRemovalReason.update() and delete() to update and delete items.

Other:

• Refactor get_request() and post_request() in class Reddit and class RequestHandler to get()/post().
• Add put() and delete() to class Reddit and class RequestHandler.
• Update return types of class RequestHandler request methods.
• Fix class Comment usage of ReactivePy. Add _async_update in place of _update for updates after fetch().
• Optimize imports.

Tests Added

Describe tests if any were written.

• test_subreddit_removal_reasons and test_subreddit_removal_reasons_add_delete.
• test_subreddit_hot to test limit in special listings.