include the HTTP proxy environment variables in PASSENV by default

[pfmoore]

Tox doesn't include HTTP proxy variables (HTTP_PROXY, HTTPS_PROXY and NO_PROXY) in PASSENV by default. As those variables basically say "you need to use this proxy or network access won't work" they seem like something that would be required for any network-related test.

I suggest that the variables are added to the list of ones included in PASSENV by default.

#258 was raised about this issue, before PASSENV existed as far as I can see, and appears to have been accepted with the resolution being that tox passed all environment variables. Presumably at some point, that was restricted again and PASSENV introduced, but the proxy variables weren't considered at that time?

[gaborbernat]

the main reason we haven't don't so yet is that you only need this if you've tests that access the network, this shouldn't be that many, not?

[pfmoore]

In general, I guess so. I hit this on pip, where the lint environment uses pre-commit and pre-commit runs git to install checks. So not a common case, certainly ;-) Conversely, though, not passing the proxy vars just leaves the test environment with broken networking, which seems pointless at best. I'm happy to say it's not a high priority, but I do think it's worth doing at some point.

[gaborbernat]

I've been setting this in all my projects, so could be the default 🤷‍♂ if you have time for a PR send it on 👍