Switch from Sentinel types to Enums

[pgjones]

The latter are much easier to work with when type hinting and can be
used successfully with mypyc, whereas the former are sadly very
difficult in both aspects.

This loses the nice property of type(NEED_DATA) is NEED_DATA (as
expanded on in the deleted docs section). However, I don't think this
is widely used in practice.

Closes #153. See also #8 for reasoning behind the introduction of the type property.

@njsmith what do you think about this?

[lovelydinosaur]

This looks to me like a breaking API change that would impact Uvicorn and HTTPX.

I do prefer the API, tho.

[Kludex]

I can check this on Uvicorn tonight. jfyk

[lovelydinosaur]

@Kludex Sure thing. Permalinks to relevant parts of code in the comment above. (Tho I can see that's not obvious.)

[Kludex]

This was merged on httpcore: encode/httpcore#579

[Kludex]

The only problem on uvicorn was:

            event = h11.InformationalResponse(
                status_code=100, headers=[], reason="Continue"
            )

Issue:

uvicorn/protocols/http/h11_impl.py:537: error: Argument "headers" to "InformationalResponse" has incompatible type "List[<nothing>]"; expected "Union[Headers, List[Tuple[bytes, bytes]], List[Tuple[str, str]]]"  [arg-type]
uvicorn/protocols/http/h11_impl.py:537: note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
uvicorn/protocols/http/h11_impl.py:537: note: Consider using "Sequence" instead, which is covariant
Found 1 error in 1 file (checked 53 source files)

I've used this branch on uvicorn to check the changes I'd need: https://github.com/encode/uvicorn/pull/1744/files