Drop URL(allow_relative=bool)

[tomchristie]

This PR pushes all url.scheme and url.host checking down into the ._transport_for_url method.

This allows us to drop the allow_relative flag on the URL class, and instead just always allow relative URLs, up until the point that we're looking up a transport instance, at which point we enforce HTTP/HTTPS.

There are two benefits here...

• allow_relative=... wasn't really ever intended as a bit of public API, we just happened to be relying on it ourselves. It's cleaner for our API for allow eg httpx.URL("") as a valid relative URL, without having to explicitly enable it. But we do still want to raise an error if you attempt to issue a request with it.
• This change is a necessary pre-requisite for Supporting alternate schemes. #954, since we don't necessarily want to hard-code enforce http/https schemes only, once we support mounting other scheme types too. Dropping the enforcement logic into the transport lookup means we'll be able to instead error if there's no appropriately mounted transport.

[florimondmanca]

Looks good! Left a few nit comments.

[florimondmanca]

(Nit) Considering using a parametrized test style? I enjoy the documentation purpose of id=..., and that each case runs as its own test...

Then should we back-port this to test_async_client.py as well?

Suggested change

	def test_get_invalid_url(server):
	with httpx.Client() as client:
	with pytest.raises(httpx.InvalidURL):
	client.get("invalid://example.org")
	with pytest.raises(httpx.InvalidURL):
	client.get("://example.org")
	with pytest.raises(httpx.InvalidURL):
	client.get("http://")
	@pytest.mark.parametrize(
	"url",
	[
	pytest.param("invalid://example.org", id="scheme-not-http(s)"),
	pytest.param("://example.org", id="no-scheme"),
	pytest.param("http://", id="no-host"),
	],
	)
	def test_get_invalid_url(server, url):
	with httpx.Client() as client:
	with pytest.raises(httpx.InvalidURL):
	client.get(url)