Add _generate_next_value_ to StrEnum (#10390)

Fixes https://github.com/python/typeshed/issues/10384

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

test_cases/stdlib/check_enum.py

@@ -0,0 +1,13 @@
+from __future__ import annotations
+
+import enum
+import sys
+from typing_extensions import Literal, assert_type
+
+if sys.version_info >= (3, 11):
+
+    class Foo(enum.StrEnum):
+        X = enum.auto()
+
+    assert_type(Foo.X, Literal[Foo.X])
+    assert_type(Foo.X.value, str)