Update sessions.py

This change will make the hierarchy request **kwargs -> Session args -> environment to be respected. This also solves problems with pip as discussed [here](pypa/pip#9691 (comment))  and  psf#5735

requests/sessions.py

@@ -527,7 +527,7 @@ def request(self, method, url,
         )
         prep = self.prepare_request(req)
 
-        proxies = proxies or {}
+        proxies = proxies or self.proxies
 
         settings = self.merge_environment_settings(
             prep.url, proxies, stream, verify, cert