Create a custom `urljoin` function that *doesn't* exclusively remove files from the URL path

[LswaN58]

The urrlib.parse.urljoin will always and forever skip over/replace any files in the path. So, e.g. when we do urljoin("http://host.of.api/path/to/app.wsgi/", "endpoint"), the "app.wsgi" part gets removed.

Need a custom function that allows us not to remove the app.wsgi part.