Rename pull_request_reviewers table to pull_request_requested_reviewers

[MoralCode]

This exists as a comment in the code left by andrew in c7364c5 three years ago.

https://github.com/chaoss/augur/blob/49a008ab97c43472339e400cb316a5323110d78d/augur/tasks/github/pull_requests/tasks.py#L70

Flagging it as an issue to fix