Fixed #27098 -- Deprecated DatabaseIntrospection.get_indexes

Thanks Akshesh <aksheshdoshi@gmail.com> for help with the PostgreSQL query.
Thanks Tim Graham for the review.

django/db/backends/sqlite3/introspection.py

@@ -1,9 +1,11 @@
 import re
+import warnings
 from collections import namedtuple
 
 from django.db.backends.base.introspection import (
     BaseDatabaseIntrospection, FieldInfo, TableInfo,
 )
+from django.utils.deprecation import RemovedInDjango21Warning
 
 field_size_re = re.compile(r'^\s*(?:var)?char\s*\(\s*(\d+)\s*\)\s*$')
 FieldInfo = namedtuple('FieldInfo', FieldInfo._fields + ('default',))
@@ -183,6 +185,10 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
         return key_columns
 
     def get_indexes(self, cursor, table_name):
+        warnings.warn(
+            "get_indexes() is deprecated in favor of get_constraints().",
+            RemovedInDjango21Warning, stacklevel=2
+        )
         indexes = {}
         for info in self._table_info(cursor, table_name):
             if info['pk'] != 0: