Replace maintenance-mode Gitdgethub with urllib3

[hugovk]

Brett wrote:

On Labour Day, @mariatta and I decided to put Gidgethub into maintenance mode; we will accept bug reports but no feature requests, PRs only by us.

https://mastodon.social/@brettcannon/117271883065229398

No rush to remove it, but we're only using it here to create headers that are used in a single Requests post.

We can:

1. Create the headers as a dict and drop Gidgethub
2. Replace Requests (which is built on top of urllib3) with the lighter urllib3