More cleanups

[pquentin]

Now that we're no longer urllib3, there are a number of things that we can and should remove right now. I think @jab, @sethmlarson and @RatanShreshtha would like to help, it turns out that those cleanups are a great way to help right now. Please send one PR for each change:

• Remove the _appveyor directory (forgot about in Run Windows tests on GitHub Actions #153)
• Remove App Engine files from src/urllib3/contrib and test/appengine and the GAE line commented out in .travis.yml
• Remove the downstream tests from CI in _travis/downstream and the references in .travis.yml
• Switch mentions in the docs from urllib3 to hip (except where we actually want to refer to urllib3)
• There's Remove unused LogRecorder class in tests #130 too which is again up-for-grabs

[pquentin]

We still need to update the docs but let's wait for #181 before working on it to avoid git conflicts.

[pquentin]

Okay, the docs are now ready to be updated 👍