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

google/auth/aws.py

@@ -424,9 +424,9 @@ def retrieve_subject_token(self, request):
 
         The logic is summarized as:
 
-        Retrieve the AWS region from the AWS_REGION environment variable or from
-        the AWS metadata server availability-zone if not found in the
-        environment variable.
+        Retrieve the AWS region from the AWS_REGION or AWS_DEFAULT_REGION
+        environment variable or from the AWS metadata server availability-zone
+        if not found in the environment variable.
 
         Check AWS credentials in environment variables. If not found, retrieve
         from the AWS metadata server security-credentials endpoint.
@@ -504,8 +504,8 @@ def retrieve_subject_token(self, request):
         )
 
     def _get_region(self, request, url):
-        """Retrieves the current AWS region from either the AWS_REGION
-        environment variable or from the AWS metadata server.
+        """Retrieves the current AWS region from either the AWS_REGION or
+        AWS_DEFAULT_REGION environment variable or from the AWS metadata server.
 
         Args:
             request (google.auth.transport.Request): A callable used to make
@@ -526,6 +526,10 @@ def _get_region(self, request, url):
         if env_aws_region is not None:
             return env_aws_region
 
+        env_aws_region = os.environ.get(environment_vars.AWS_DEFAULT_REGION)
+        if env_aws_region is not None:
+            return env_aws_region
+
         if not self._region_url:
             raise exceptions.RefreshError("Unable to determine AWS region")
         response = request(url=self._region_url, method="GET")

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"

tests/test_aws.py

@@ -1043,6 +1043,56 @@ def test_retrieve_subject_token_success_environment_vars(self, utcnow, monkeypat
             }
         )
 
+    @mock.patch("google.auth._helpers.utcnow")
+    def test_retrieve_subject_token_success_environment_vars_with_default_region(
+        self, utcnow, monkeypatch
+    ):
+        monkeypatch.setenv(environment_vars.AWS_ACCESS_KEY_ID, ACCESS_KEY_ID)
+        monkeypatch.setenv(environment_vars.AWS_SECRET_ACCESS_KEY, SECRET_ACCESS_KEY)
+        monkeypatch.setenv(environment_vars.AWS_SESSION_TOKEN, TOKEN)
+        monkeypatch.setenv(environment_vars.AWS_DEFAULT_REGION, self.AWS_REGION)
+        utcnow.return_value = datetime.datetime.strptime(
+            self.AWS_SIGNATURE_TIME, "%Y-%m-%dT%H:%M:%SZ"
+        )
+        credentials = self.make_credentials(credential_source=self.CREDENTIAL_SOURCE)
+
+        subject_token = credentials.retrieve_subject_token(None)
+
+        assert subject_token == self.make_serialized_aws_signed_request(
+            {
+                "access_key_id": ACCESS_KEY_ID,
+                "secret_access_key": SECRET_ACCESS_KEY,
+                "security_token": TOKEN,
+            }
+        )
+
+    @mock.patch("google.auth._helpers.utcnow")
+    def test_retrieve_subject_token_success_environment_vars_with_both_regions_set(
+        self, utcnow, monkeypatch
+    ):
+        monkeypatch.setenv(environment_vars.AWS_ACCESS_KEY_ID, ACCESS_KEY_ID)
+        monkeypatch.setenv(environment_vars.AWS_SECRET_ACCESS_KEY, SECRET_ACCESS_KEY)
+        monkeypatch.setenv(environment_vars.AWS_SESSION_TOKEN, TOKEN)
+        monkeypatch.setenv(environment_vars.AWS_DEFAULT_REGION, "Malformed AWS Region")
+        # This test makes sure that the AWS_REGION gets used over AWS_DEFAULT_REGION,
+        # So, AWS_DEFAULT_REGION is set to something that would cause the test to fail,
+        # And AWS_REGION is set to the a valid value, and it should succeed
+        monkeypatch.setenv(environment_vars.AWS_REGION, self.AWS_REGION)
+        utcnow.return_value = datetime.datetime.strptime(
+            self.AWS_SIGNATURE_TIME, "%Y-%m-%dT%H:%M:%SZ"
+        )
+        credentials = self.make_credentials(credential_source=self.CREDENTIAL_SOURCE)
+
+        subject_token = credentials.retrieve_subject_token(None)
+
+        assert subject_token == self.make_serialized_aws_signed_request(
+            {
+                "access_key_id": ACCESS_KEY_ID,
+                "secret_access_key": SECRET_ACCESS_KEY,
+                "security_token": TOKEN,
+            }
+        )
+
     @mock.patch("google.auth._helpers.utcnow")
     def test_retrieve_subject_token_success_environment_vars_no_session_token(
         self, utcnow, monkeypatch