Fix another new test on Python 3.8

Author: tulir

mautrix/crypto/attachments/async_attachments_test.py

@@ -5,6 +5,8 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
+from __future__ import annotations
+
 from mautrix.types import EncryptedFile
 
 from .async_attachments import async_encrypt_attachment, async_inplace_encrypt_attachment