UvicornWorker 'auto' loop and http implementations don't work on Linux without extras installed

[mdgilene]

Checklist

• The bug is reproducible against the latest release and/or master.
• There are no similar issues or pull requests to fix it yet.

Describe the bug

After #666 , uvloop and and httptools are no longer installed unless you specify to install the 'standard' extra. Because of this when running with loop='auto' and http='auto' the application fails to start due to 'uvloop' and 'httptools' being selected. NOTE: This only happens when running the uvicorn.workers.UvicornWorker through something like Gunicorn.

To reproduce

pip install uvicorn gunicorn

#main.py
async def app(scope, receive, send):
    assert scope['type'] == 'http'

    await send({
        'type': 'http.response.start',
        'status': 200,
        'headers': [
            [b'content-type', b'text/plain'],
        ],
    })
    await send({
        'type': 'http.response.body',
        'body': b'Hello, world!',
    })

gunicorn -k uvicorn.workers.UvicornWorker main:app

Expected behavior

'auto' should pick 'asyncio' and 'h11' for their respective settings on Linux if the 'standard' extra is not installed.

Actual behavior

Application starts up but immediately throws ModuleNotFoundError: No module named 'uvloop'. If you install uvloop it then complains about not finding httptools. After installing both, no more issues occur.

Debugging material

Environment

• Running uvicorn 0.12.1 with CPython 3.7.6 on Linux

Additional context

[euri10]

Yep, we should be selecting good defaults

[florimondmanca]

For anyone passing by a looking for a workaround…

https://www.uvicorn.org/deployment/#gunicorn

# myproject/workers.py
class MyUvicornWorker(UvicornWorker):
    CONFIG_KWARGS = {"loop": "auto", "http": "auto"}

$ gunicorn -w myproject.workers.MyUvicornWorker app:app

See #266