Factor out AWS_STS_REGIONAL_ENDPOINTS to allow customization (#3257)

botocore/configprovider.py

@@ -144,12 +144,6 @@
         'auto',
         None,
     ),
-    'sts_regional_endpoints': (
-        'sts_regional_endpoints',
-        'AWS_STS_REGIONAL_ENDPOINTS',
-        'legacy',
-        None,
-    ),
     'retry_mode': ('retry_mode', 'AWS_RETRY_MODE', 'legacy', None),
     'defaults_mode': ('defaults_mode', 'AWS_DEFAULTS_MODE', 'legacy', None),
     # We can't have a default here for v1 because we need to defer to
@@ -175,6 +169,28 @@
         None,
     ),
 }
+
+# Evaluate AWS_STS_REGIONAL_ENDPOINTS settings
+try:
+    # This is not a public interface and is subject to abrupt breaking changes.
+    # Any usage is not advised or supported in external code bases.
+    from botocore.customizations.sts import (
+        sts_default_setting as _sts_default_setting,
+    )
+except ImportError:
+    _sts_default_setting = 'legacy'
+
+_STS_DEFAULT_SETTINGS = {
+    'sts_regional_endpoints': (
+        'sts_regional_endpoints',
+        'AWS_STS_REGIONAL_ENDPOINTS',
+        _sts_default_setting,
+        None,
+    ),
+}
+BOTOCORE_DEFAUT_SESSION_VARIABLES.update(_STS_DEFAULT_SETTINGS)
+
+
 # A mapping for the s3 specific configuration vars. These are the configuration
 # vars that typically go in the s3 section of the config file. This mapping
 # follows the same schema as the previous session variable mapping.