Clear usage errors when incorrect auth packages are applied.

[tomchristie]

Prompted by this issue... #1558

It's a fairly easy user error for users to attempt to apply a requests auth package to an httpx client. Understandable that folks might try to do that. Question is, could this be something worth handling with clearer error messaging (or not)?

Note that the traceback on that issue only fails as a result of duck typing. It'd be clearer if we raised a TypeError at the point an invalid auth is applied to the client...

client.auth = ...  # Raises type error unless auth is a subclass of `httpx.Auth`.

Of course there's a wider question that this raises regarding type checking on our interfaces as a whole, when to provide for strict TypeError raising, and when to just suck it up and expect our users to use the package as intended. Trade off being that adding stricter type checking brings a little extra gumpf into our codebase, which makes it a little more complex for users digging under the hood perhaps?