fix: aerich upgrade raises OperationalError when unique constraint dropped at migration 1_xxx.py with postgres (#383)

aerich/ddl/postgres/__init__.py

@@ -10,7 +10,7 @@ class PostgresDDL(BaseDDL):
     schema_generator_cls = AsyncpgSchemaGenerator
     DIALECT = AsyncpgSchemaGenerator.DIALECT
     _ADD_INDEX_TEMPLATE = 'CREATE {unique}INDEX "{index_name}" ON "{table_name}" ({column_names})'
-    _DROP_INDEX_TEMPLATE = 'DROP INDEX "{index_name}"'
+    _DROP_INDEX_TEMPLATE = 'DROP INDEX IF EXISTS "{index_name}"'
     _ALTER_NULL_TEMPLATE = 'ALTER TABLE "{table_name}" ALTER COLUMN "{column}" {set_drop} NOT NULL'
     _MODIFY_COLUMN_TEMPLATE = (
         'ALTER TABLE "{table_name}" ALTER COLUMN "{column}" TYPE {datatype}{using}'