Added pproxy test fixture, which allows to test requesting through socks proxy

[cdeler]

As a part of https://github.com/encode/httpx/issues/203 (draft pr is #188) I'd like to introduce the "socks" test fixture.

Using pproxy it starts a subprocess with socks4 and socks5 (with auth and without auth) proxies on localhost.

Also the PR contains some new types, required in the fixture

There is a problem: this server doesn't support socks4a proxy (may be it's not so important)

[lovelydinosaur]

I don't think this is quite what I was expecting here.
A more minimal first step would be adding some pproxy tests cases for our existing regular HTTP/HTTPS proxy support.

[cdeler]

@tomchristie

You are talking about proxy_server fixture, which is used by test_http_proxy and test_proxy_https_requests? (if so, I'm doing that)

httpcore/tests/conftest.py

Lines 84 to 85 in 1f67394

	@pytest.fixture()
	defproxy_server(example_org_cert_path: str) ->typing.Iterator[URL]:

[cdeler]

Closing this PR, replacing it by #199