Fix urllib3 mocking to conform to current API

[Perfect5th]

The expected responses from requests requests are now instances of urllib3.response.HTTPResponse, not http.client.HTTPResponse. This change both fixes that, and ensures the response can be read by fixing the incorrect HTTP message (it was missing a CRLF). I also added a Content-Type header because sometimes the request body does not get read if this header is absent.

tox should pass for py38 and py310. I didn't test py35 and py36 as both are now EOL.

fixes #601

[Perfect5th]

New rev with fixes for 3.5 and 3.6 - turns out the old behaviour of the mock was what they wanted. Proof tox is passing for these versions: https://github.com/Perfect5th/talisker/actions/runs/5438859830/jobs/9890351211

Will work on other failing CI bits soon (docs, etc)

[Perfect5th]

Bumping the Sphinx version gets the docs building again (see https://github.com/Perfect5th/talisker/actions/runs/5439103405), so this now also closes #599 and therefore closes #600

[wgrant]

Thanks, LGTM now.

[verterok]

Looks like this was left behind, sorry for missing it.
I just tried to run the CI jobs and looks like one dependency needs to be updated:

diff --git a/requirements.tests.txt b/requirements.tests.txt
index d36d68c..2765aa8 100644
--- a/requirements.tests.txt
+++ b/requirements.tests.txt
@@ -16,7 +16,7 @@ setuptools==44.0.0;python_version<"3.10"
 setuptools>64;python_version>="3.10"
 coverage==5.0.3;python_version<"3.10"
 coverage>=6;python_version>="3.10"
-flaky==3.6.1
+flaky==3.8.1

 # for integration tests
 # eventlet is pinned until https://github.com/benoitc/gunicorn/pull/2581

If you want to include the change in your PR, it should land (tested locally)

[Perfect5th]

I've added the suggested flaky dependency version change to the PR, thanks @verterok !