create a sans-io docker client class and impliment Client with blocking requests

use https://github.com/mikeal/deferred to create a sans-io version of Client (SansIOClient) that requires something like:

class SimpleStream(object):
    def next(self) -> Deferred:
        ...

class IOAdapter(object):
    def request(self, **kwargs) -> Deferred:
        ...

    def stream(self, **kwargs) -> SimpleStream:
        ...

    def unwrap_deferred(self, deferred: Deferred) -> Any:
        ...

and then implement it with something like:

class BlockingSimpleStream(SimpleStream):
    def __init__(self, stream):
        self.generator = _stream_helper(stream):
    def next(self) -> Deferred:
        return deferred.succeeded(next(self.generator))

class BlockingIOAdapter(IOAdapter):
    def __init__(session: requests.Session):
        self.session = session

    def request(self, **kwargs) -> Deferred:
        return deferred.execute(self.session.request, **kwargs)

    def stream(self, **kwargs) -> BlockingSimpleStream:
        return BlockingSimpleStream(self.session.request(**kwargs, stream=True))

    def unwrap_deferred(self, d: Deferred):
        return deferred.waitForDeferred(d).getResult()

Start by reading the existing Client entry point and its requests.Session integration, then compare them with the proposed SansIOClient, SimpleStream, and IOAdapter interfaces. Done means the sans-IO client and blocking implementation exist with deferred request, streaming, and unwrapping behavior as described; no tests or filenames are mentioned in the issue.