feat: allow the AWS_DEFAULT_REGION environment variable (#721)
Amazon has this variable documented, and apparently people are trying to
use it, so we should support it
diff --git a/google/auth/environment_vars.py b/google/auth/environment_vars.py
index 416bab0..f027741 100644
--- a/google/auth/environment_vars.py
+++ b/google/auth/environment_vars.py
@@ -69,3 +69,4 @@
AWS_SECRET_ACCESS_KEY = "AWS_SECRET_ACCESS_KEY"
AWS_SESSION_TOKEN = "AWS_SESSION_TOKEN"
AWS_REGION = "AWS_REGION"
+AWS_DEFAULT_REGION = "AWS_DEFAULT_REGION"