Running httpcore on MicroPython.

[tomchristie]

I'm curious what blockers we've got to being able to run httpcore on MicroPython? ☺️

I suppose the place to start here is...

• A review of which imports we currently use.
• A comparison against which modules the micropython environment provides.

(Also note that this will differ for httpcore[asyncio] and httpcore[trio].)