cf #327 -- if a user is behind a proxy, the current "recommended" solution is a moderately hideous hack involving copy-pasting your own HttpClient class

https://stackoverflow.com/questions/43597379/how-to-provide-proxy-information-to-twilio-api-with-python/43608637#43608637

requests itself works quite happily with proxy servers by auto-detecting the http_proxy and https_proxy environment variables, it would be nice if that still worked for requests-as-used-by-twilio.

(disclosure: i work at pythonanywhere; our free users have to go via our proxy for internet access. we set the standard env vars, so this "just works" for requests, and most 3rd party api tools, but twilio is one of the exceptions.)