Consistent handling of HTTP/S proxy environment variables

[davidwegs]

Consistent handling of HTTP/S proxy environment variables

Description

This PR provides consistent handling of the http_proxy, https_proxy, and no_proxy environment variables in line with the behavior of the Requests python library. Previously no_proxy wasn't being respected (#2077).

This is done by removing explicit handling of the http_proxy and https_proxy env variables in favor of downstream handling through the Requests library. Explicitly provided proxy settings through the proxy_url argument to the connection class and through the set_http_proxy method are still supported and still take precedence over the environment variables.

Status

done, ready for review

Checklist (tick everything that applies)

• [x] Code linting (required, can be done after the PR checks)
• [ ] Documentation
• [x] Tests
• [ ] ICLA (required for bigger changes)