Propagate path prefix in server URLs to all requests

[Kleinjohann]

Summary of the changes / Why this is an improvement

Path prefixes in server URLs are now propagated to all requests.
E.g. if the server URL 'http://my-server:1234/crate/' is supplied, requests with SQL queries will be sent to 'http://my-server:1234/crate/_sql?types=true'. Currently, the prefix would be ignored, resulting in 'http://my-server:1234/_sql?types=true' for the same server URL input.
I've encountered multiple setups with proxies that handle forwarding based on path prefixes. This change makes the client work in such setups.

One aspect I'm unsure about:

crate-python/tests/client/test_http.py

Lines 238 to 239 in 7ec1b56

	with patch("crate.client.http.urlparse", side_effect=Exception):
	Client("")

Seems like you don't want the client to crash if urllib.parse.urlparse throws an exception, so I wrapped my call in a try-except block:
https://github.com/Kleinjohann/crate-python/blob/d5a7902f44f920646dfe0b19884fc0a635879d19/src/crate/client/http.py#L147-L155
This might however just move unavoidable errors to later stages where they will result in less meaningful error messages. The only hint to anyone trying to debug this would be the warning in the log.
I'm probably missing something here, and it's fine for me to keep it as is, I just somehow didn't feel comfortable implementing it like this.

Fixes #743

[mfussenegger]

Could you also add a note to the CHANGES.rst?

Change looks good to me otherwise.

[mfussenegger]

Could you move the .strip("/") for the path_prefix to the CTOR to only do that once per Server instantiation instead of once per request?

[Kleinjohann]

Good point, sorry for missing that, I moved the .strip("/").

[Kleinjohann]

Thanks for your review!
I moved the .strip("/") call, added a bullet point to CHANGES.rst, and rebased to resolve the conflict.