PyPI extension search not functional with lowercase http_proxy environment variable.

[jgoodson]

Description

Even after #15219, extension lookup in proxy-requiring environments fail if only the lower-case http_proxy variant is provided. Although there is no standard for these variables, the lower-case variant is "more standard", working in most major tools that handle these environments. For reference, see https://about.gitlab.com/blog/2021/01/27/we-need-to-talk-no-proxy/#http_proxy-and-https_proxy for a discussion of these variables.

While you might expect environment variables to be all-caps, http_proxy came first, so that's the de facto standard. When in doubt, go with the lowercase form because that's universally supported.

Reproduce

Start up Jupyter Lab in an environment which requires an HTTP proxy to connect, with that proxy URL set in http_proxy.

Expected behavior

Lab connects to PyPI through the proxy.

Very simple fix

All that needs to be done is to add http_proxy/https_proxy to the logic for obtaining a proxy URL. The previously-linked discussion from the Gitlab engineering blog has some strong recommendations for the use (or lack of) of the uppercase form. Since Lab is already using the uppercase form, it could potentially break things if removed.

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