Fix psycopg2.sql.SQL.join annotation

Fixes python#10715.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

stubs/psycopg2/psycopg2/sql.pyi

@@ -1,4 +1,3 @@
-from _typeshed import SupportsIter
 from collections.abc import Iterable, Iterator
 from typing import Any, Generic, TypeVar
 
@@ -27,7 +26,7 @@ class SQL(Composable):
     @property
     def string(self) -> str: ...
     def format(self, *args: Composable, **kwargs: Composable) -> Composed: ...
-    def join(self, seq: SupportsIter[Composable]) -> Composed: ...
+    def join(self, seq: Iterable[Composable]) -> Composed: ...
 
 class Identifier(Composable):
     def __init__(self, *strings: str) -> None: ...