Fix bearer auth for bluesky

[GiovanH]

@mikf I'm sure you'll want to reorganize this, but this fixes #6134 and makes bluesky work, instead of not working.

Due to under-the-hood implementation of the requests library, we have to pass the bearer authentication info in the special auth kwarg, or else the basic token from the session will overwrite it.

[mikf]

Pretty sure all of this can be accomplished with one line of code:

    self.session.auth = None

[GiovanH]

I was acting under the assumption that some pre-bearer requests in the session still needed the Basic token. If, as per #6134 (comment), we can remove the basic token entirely, self.session.auth = None would do it. But we need to figure out the basic token thing from #6134 first