support plugable IO with anyio and plugable http client strategy

[graingert]

I'd like be able to use httpio with httpx on trio, I'd be able to do this if all of this with a pluggable session strategy:

@attr.s(frozen=True)
class AIOHttpFactory:
    _session = attr.ib()
    _kwargs = attr.ib()

    def length(self, url) -> int:
        async with self._session.head(self.url, **self._kwargs) as response:
            response.raise_for_status()
            return int(response.headers.get('content-length', None))

    def get(self, url, start, end) -> bytes:
        headers = {
            "Range": "bytes=%d-%d" % (start, end - 1),
            **self._kwargs.get("headers", {})
        }
        kwargs = {**self._kwargs, "headers": headers}
        async with self._session.get(url, **kwargs) as response:
            response.raise_for_status()
            return await response.read()

    @classmethod
    @contextlib.asynccontextmanager
    def session(cls, kwargs):
        async with asyncio.ClientSession() as s:
            yield cls(s, kwargs)

    def __init__(self, url, block_size=-1, session_factory=None, **kwargs):
        """
        :param url: The URL of the file to open
        :param block_size: The cache block size, or `-1` to disable caching.
        :param kwargs: Additional arguments to pass to `session.get`
        """
        super(AsyncHTTPIOFile, self).__init__()
        self.url = url
        self.block_size = block_size

        if session_factory is None
            from . import aiohttp_strategy
            session_factory = aiohttp_strategy.AIOHttpFactory

        self._session_factory = session_factory
        ...

[graingert]

@barneygale If you like the concept I'll work on a PR, and switch the tests to use pytest-anyio and add an HTTPXHttpFactory