Alternative client (httpx) support

[johtso]

I've been exploring async http client options and in the process have found that by leaving requests+CacheControl behind, there seemed to be no options that provided ETag based caching.

In order to get caching functionality with httpx, which seems like one of the most solid async clients at the moment, it looks like all that would be needed is a Transport.

Would making this project more client agnostic be a reasonable goal?

From a brief look around it seems like there are a couple of places outside of adapter.py that requests is imported, and requests is currently required in setup.py
Thoughts?