Recently faced an issue with a proxy responding in lowercase http, which caused this:

ProtocolError('Connection aborted.', BadStatusLine('http/1.1 200 connection established\r\n',))

Changing the string to uppercase before checking if it starts with HTTP fixes this issue and allows to use this proxy.

I know that the proxy is at fault here, but seeing as other applications (web browsers, office suite, text editors, etc.) still work behind this proxy, I think it might be a reasonable fix to have...