Pass through *_proxy environment variables by default

[pytoxbot]

• Bitbucket: https://bitbucket.org/hpk42/tox/issue/258
• Originally reported by: serenico
• Originally created at: 2015-06-17T16:33:53.320

Inside corporations is quite common being working beside a proxy. In Linux some applications (like pip) takes proxy configuration from following environment variables:

http_proxy https_proxy no_proxy
HTTP_PROXY HTTPS_PROXY NO_PROXY

Being variables of the system and quite standard and not being specific of projects, I think they should be passed through by default.

[pytoxbot]

Original comment by @hpk42

with https://bitbucket.org/hpk42/tox/commits/521459223e524337d187ef07277f0ee1485cd313 all env variables are passed through now.

[pytoxbot]

Original comment by @stefano-m

I think it makes sense. Windows systems may benefit too as some applications (e.g. git or node) honor the proxy environment variables when they're set.