Use environment markers in setup.py

[songww]

Use environment markers in setup.py rather than explicitly switching requirements. #135

uvloop: https://github.com/MagicStack/uvloop/blob/8d5a9a3a4a5a74b1caad12423305b23049c00ab9/setup.py#L241

httptools: https://github.com/MagicStack/httptools/blob/14043bd7cb46fb879aaa1072dccf9a3817afaf32/setup.py#L22

[tomchristie]

Okay, I'm undecided on this just now.
It's possible that we'll make a different jump with 0.4, and instead force either pip install uvicorn for minimal dependencies, or pip install uvicorn[full] to include all optional dependencies.