I am currently working on #178 and extended the tests for the proxy selection mechanism, especially when the env variables like HTTP_PROXY and HTTPS_PROXY are present. One thing that I've observed is the fallback behavior:

• no ENV vars: HTTPS DSNs can fallback to HTTP proxies (see test)
• only HTTP_PROXY env var: no fallback for HTTPS DSNs, i.e. no proxy is auto-selected

Is that behavior intended or a bug? In either case, my PR for #178 will also include tests (and potential fixes) for that.