Concrete return type for SymbolTable.get_identifiers (#8054)

Author: pamelafox

stdlib/symtable.pyi

@@ -1,4 +1,5 @@
 import sys
+from _collections_abc import dict_keys
 from collections.abc import Sequence
 from typing import Any
 
@@ -18,7 +19,7 @@ class SymbolTable:
     if sys.version_info < (3, 9):
         def has_exec(self) -> bool: ...
 
-    def get_identifiers(self) -> Sequence[str]: ...
+    def get_identifiers(self) -> dict_keys[str, int]: ...
     def lookup(self, name: str) -> Symbol: ...
     def get_symbols(self) -> list[Symbol]: ...
     def get_children(self) -> list[SymbolTable]: ...