Add a comment about lowercase env. variables precedence over uppercase
diff --git a/requests/utils.py b/requests/utils.py
index 3f50d485d9..07f8c7f764 100644
--- a/requests/utils.py
+++ b/requests/utils.py
@@ -696,6 +696,8 @@ def should_bypass_proxies(url, no_proxy):
 
     :rtype: bool
     """
+    # Prioritize lowercase environment variables over uppercase
+    # to keep a consistent behaviour with other http projects (curl, wget).
     get_proxy = lambda k: os.environ.get(k) or os.environ.get(k.upper())
 
     # First check whether no_proxy is defined. If it is, check that the URL

As suggested by @sigmavirus24 for the follow-up of issue #4579 