Use blockbuster to detect blocking calls in the asyncio loop

[cbornet]

BlockBuster is a library to detect harmful blocking calls done in the async event loop.
It is just a few lines to integrate with pytest.
Would you consider using it in httpx ?

[cbornet]

I have a PR ready if you're interested.
Globally, httpx is a good citizen and the only blocking call it detected is mimetypes.guess_type in Request constructor. mimetypes has a global cache so this blocking call is only done once.
Solutions for this would be:

• defer the mimetype resolution to when the request is actually called and call mimetypes.guess_type in a thread
• or just accept that it blocks for a short time only once and add an exemption in blockbuster setup.
  I've also seen blocking calls made when using HTTPS but there seems to be no unit tests for async+TLS.

[cbornet]

Since I had it ready, I opened #3486 . Feel free to close if not interested.